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TensorDictModule

In this tutorial you will learn how to use TensorDictModule and TensorDictSequential to create generic and reusable modules that can accept TensorDict as input.

For a convenient usage of the TensorDict class with nn.Module, tensordict provides an interface between the two named TensorDictModule. The TensorDictModule class is an nn.Module that takes a TensorDict as input when called. It is up to the user to define the keys to be read as input and output.

TensorDictModule by examples

import torch
import torch.nn as nn
from tensordict import TensorDict
from tensordict.nn import TensorDictModule, TensorDictSequential

Example 1: Simple usage

We have a TensorDict with 2 entries "a" and "b" but only the value associated with "a" has to be read by the network.

tensordict = TensorDict(
    {"a": torch.randn(5, 3), "b": torch.zeros(5, 4, 3)},
    batch_size=[5],
)
linear = TensorDictModule(nn.Linear(3, 10), in_keys=["a"], out_keys=["a_out"])
linear(tensordict)
assert (tensordict.get("b") == 0).all()
print(tensordict)
TensorDict(
    fields={
        a: Tensor(shape=torch.Size([5, 3]), device=cpu, dtype=torch.float32, is_shared=False),
        a_out: Tensor(shape=torch.Size([5, 10]), device=cpu, dtype=torch.float32, is_shared=False),
        b: Tensor(shape=torch.Size([5, 4, 3]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([5]),
    device=None,
    is_shared=False)

Example 2: Multiple inputs

Suppose we have a slightly more complex network that takes 2 entries and averages them into a single output tensor. To make a TensorDictModule instance read multiple input values, one must register them in the in_keys keyword argument of the constructor.

class MergeLinear(nn.Module):
    def __init__(self, in_1, in_2, out):
        super().__init__()
        self.linear_1 = nn.Linear(in_1, out)
        self.linear_2 = nn.Linear(in_2, out)

    def forward(self, x_1, x_2):
        return (self.linear_1(x_1) + self.linear_2(x_2)) / 2
tensordict = TensorDict(
    {
        "a": torch.randn(5, 3),
        "b": torch.randn(5, 4),
    },
    batch_size=[5],
)

mergelinear = TensorDictModule(
    MergeLinear(3, 4, 10), in_keys=["a", "b"], out_keys=["output"]
)

mergelinear(tensordict)
TensorDict(
    fields={
        a: Tensor(shape=torch.Size([5, 3]), device=cpu, dtype=torch.float32, is_shared=False),
        b: Tensor(shape=torch.Size([5, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        output: Tensor(shape=torch.Size([5, 10]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([5]),
    device=None,
    is_shared=False)

Example 3: Multiple outputs

Similarly, TensorDictModule not only supports multiple inputs but also multiple outputs. To make a TensorDictModule instance write to multiple output values, one must register them in the out_keys keyword argument of the constructor.

class MultiHeadLinear(nn.Module):
    def __init__(self, in_1, out_1, out_2):
        super().__init__()
        self.linear_1 = nn.Linear(in_1, out_1)
        self.linear_2 = nn.Linear(in_1, out_2)

    def forward(self, x):
        return self.linear_1(x), self.linear_2(x)
tensordict = TensorDict({"a": torch.randn(5, 3)}, batch_size=[5])

splitlinear = TensorDictModule(
    MultiHeadLinear(3, 4, 10),
    in_keys=["a"],
    out_keys=["output_1", "output_2"],
)
splitlinear(tensordict)
TensorDict(
    fields={
        a: Tensor(shape=torch.Size([5, 3]), device=cpu, dtype=torch.float32, is_shared=False),
        output_1: Tensor(shape=torch.Size([5, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        output_2: Tensor(shape=torch.Size([5, 10]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([5]),
    device=None,
    is_shared=False)

When having multiple input keys and output keys, make sure they match the order in the module.

TensorDictModule can work with TensorDict instances that contain more tensors than what the in_keys attribute indicates.

Unless a vmap operator is used, the TensorDict is modified in-place.

Ignoring some outputs

Note that it is possible to avoid writing some of the tensors to the TensorDict output, using "_" in out_keys.

Example 4: Combining multiple TensorDictModule with TensorDictSequential

To combine multiple TensorDictModule instances, we can use TensorDictSequential. We create a list where each TensorDictModule must be executed sequentially. TensorDictSequential will read and write keys to the tensordict following the sequence of modules provided.

We can also gather the inputs needed by TensorDictSequential with the in_keys property, and the outputs keys are found at the out_keys attribute.

tensordict = TensorDict({"a": torch.randn(5, 3)}, batch_size=[5])

splitlinear = TensorDictModule(
    MultiHeadLinear(3, 4, 10),
    in_keys=["a"],
    out_keys=["output_1", "output_2"],
)
mergelinear = TensorDictModule(
    MergeLinear(4, 10, 13),
    in_keys=["output_1", "output_2"],
    out_keys=["output"],
)

split_and_merge_linear = TensorDictSequential(splitlinear, mergelinear)

assert split_and_merge_linear(tensordict)["output"].shape == torch.Size([5, 13])

Do’s and don’t with TensorDictModule

Don’t use nn.Sequence, similar to nn.Module, it would break features such as functorch compatibility. Do use TensorDictSequential instead.

Don’t assign the output tensordict to a new variable, as the output tensordict is just the input modified in-place:

tensordict = module(tensordict) # ok!

tensordict_out = module(tensordict) # don’t!

ProbabilisticTensorDictModule

ProbabilisticTensorDictModule is a non-parametric module representing a probability distribution. Distribution parameters are read from tensordict input, and the output is written to an output tensordict. The output is sampled given some rule, specified by the input default_interaction_type argument and the exploration_mode() global function. If they conflict, the context manager precedes.

It can be wired together with a TensorDictModule that returns a tensordict updated with the distribution parameters using ProbabilisticTensorDictSequential. This is a special case of TensorDictSequential that terminates in a ProbabilisticTensorDictModule.

ProbabilisticTensorDictModule is responsible for constructing the distribution (through the get_dist() method) and/or sampling from this distribution (through a regular __call__() to the module). The same get_dist() method is exposed on ``ProbabilisticTensorDictSequential.

One can find the parameters in the output tensordict as well as the log probability if needed.

from tensordict.nn import (
    ProbabilisticTensorDictModule,
    ProbabilisticTensorDictSequential,
)
from tensordict.nn.distributions import NormalParamExtractor
from torch import distributions as dist

td = TensorDict({"input": torch.randn(3, 4), "hidden": torch.randn(3, 8)}, [3])
net = torch.nn.GRUCell(4, 8)
net = TensorDictModule(net, in_keys=["input", "hidden"], out_keys=["hidden"])
extractor = NormalParamExtractor()
extractor = TensorDictModule(extractor, in_keys=["hidden"], out_keys=["loc", "scale"])
td_module = ProbabilisticTensorDictSequential(
    net,
    extractor,
    ProbabilisticTensorDictModule(
        in_keys=["loc", "scale"],
        out_keys=["action"],
        distribution_class=dist.Normal,
        return_log_prob=True,
    ),
)
print(f"TensorDict before going through module: {td}")
td_module(td)
print(f"TensorDict after going through module now as keys action, loc and scale: {td}")
TensorDict before going through module: TensorDict(
    fields={
        hidden: Tensor(shape=torch.Size([3, 8]), device=cpu, dtype=torch.float32, is_shared=False),
        input: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([3]),
    device=None,
    is_shared=False)
TensorDict after going through module now as keys action, loc and scale: TensorDict(
    fields={
        action: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        hidden: Tensor(shape=torch.Size([3, 8]), device=cpu, dtype=torch.float32, is_shared=False),
        input: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        loc: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        sample_log_prob: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False),
        scale: Tensor(shape=torch.Size([3, 4]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([3]),
    device=None,
    is_shared=False)

Showcase: Implementing a transformer using TensorDictModule

To demonstrate the flexibility of TensorDictModule, we are going to create a transformer that reads TensorDict objects using TensorDictModule.

The following figure shows the classical transformer architecture (Vaswani et al, 2017).

The transformer png

We have let the positional encoders aside for simplicity.

Let’s re-write the classical transformers blocks:

class TokensToQKV(nn.Module):
    def __init__(self, to_dim, from_dim, latent_dim):
        super().__init__()
        self.q = nn.Linear(to_dim, latent_dim)
        self.k = nn.Linear(from_dim, latent_dim)
        self.v = nn.Linear(from_dim, latent_dim)

    def forward(self, X_to, X_from):
        Q = self.q(X_to)
        K = self.k(X_from)
        V = self.v(X_from)
        return Q, K, V


class SplitHeads(nn.Module):
    def __init__(self, num_heads):
        super().__init__()
        self.num_heads = num_heads

    def forward(self, Q, K, V):
        batch_size, to_num, latent_dim = Q.shape
        _, from_num, _ = K.shape
        d_tensor = latent_dim // self.num_heads
        Q = Q.reshape(batch_size, to_num, self.num_heads, d_tensor).transpose(1, 2)
        K = K.reshape(batch_size, from_num, self.num_heads, d_tensor).transpose(1, 2)
        V = V.reshape(batch_size, from_num, self.num_heads, d_tensor).transpose(1, 2)
        return Q, K, V


class Attention(nn.Module):
    def __init__(self, latent_dim, to_dim):
        super().__init__()
        self.softmax = nn.Softmax(dim=-1)
        self.out = nn.Linear(latent_dim, to_dim)

    def forward(self, Q, K, V):
        batch_size, n_heads, to_num, d_in = Q.shape
        attn = self.softmax(Q @ K.transpose(2, 3) / d_in)
        out = attn @ V
        out = self.out(out.transpose(1, 2).reshape(batch_size, to_num, n_heads * d_in))
        return out, attn


class SkipLayerNorm(nn.Module):
    def __init__(self, to_len, to_dim):
        super().__init__()
        self.layer_norm = nn.LayerNorm((to_len, to_dim))

    def forward(self, x_0, x_1):
        return self.layer_norm(x_0 + x_1)


class FFN(nn.Module):
    def __init__(self, to_dim, hidden_dim, dropout_rate=0.2):
        super().__init__()
        self.FFN = nn.Sequential(
            nn.Linear(to_dim, hidden_dim),
            nn.ReLU(),
            nn.Linear(hidden_dim, to_dim),
            nn.Dropout(dropout_rate),
        )

    def forward(self, X):
        return self.FFN(X)


class AttentionBlock(nn.Module):
    def __init__(self, to_dim, to_len, from_dim, latent_dim, num_heads):
        super().__init__()
        self.tokens_to_qkv = TokensToQKV(to_dim, from_dim, latent_dim)
        self.split_heads = SplitHeads(num_heads)
        self.attention = Attention(latent_dim, to_dim)
        self.skip = SkipLayerNorm(to_len, to_dim)

    def forward(self, X_to, X_from):
        Q, K, V = self.tokens_to_qkv(X_to, X_from)
        Q, K, V = self.split_heads(Q, K, V)
        out, attention = self.attention(Q, K, V)
        out = self.skip(X_to, out)
        return out


class EncoderTransformerBlock(nn.Module):
    def __init__(self, to_dim, to_len, latent_dim, num_heads):
        super().__init__()
        self.attention_block = AttentionBlock(
            to_dim, to_len, to_dim, latent_dim, num_heads
        )
        self.FFN = FFN(to_dim, 4 * to_dim)
        self.skip = SkipLayerNorm(to_len, to_dim)

    def forward(self, X_to):
        X_to = self.attention_block(X_to, X_to)
        X_out = self.FFN(X_to)
        return self.skip(X_out, X_to)


class DecoderTransformerBlock(nn.Module):
    def __init__(self, to_dim, to_len, from_dim, latent_dim, num_heads):
        super().__init__()
        self.attention_block = AttentionBlock(
            to_dim, to_len, from_dim, latent_dim, num_heads
        )
        self.encoder_block = EncoderTransformerBlock(
            to_dim, to_len, latent_dim, num_heads
        )

    def forward(self, X_to, X_from):
        X_to = self.attention_block(X_to, X_from)
        X_to = self.encoder_block(X_to)
        return X_to


class TransformerEncoder(nn.Module):
    def __init__(self, num_blocks, to_dim, to_len, latent_dim, num_heads):
        super().__init__()
        self.encoder = nn.ModuleList(
            [
                EncoderTransformerBlock(to_dim, to_len, latent_dim, num_heads)
                for i in range(num_blocks)
            ]
        )

    def forward(self, X_to):
        for i in range(len(self.encoder)):
            X_to = self.encoder[i](X_to)
        return X_to


class TransformerDecoder(nn.Module):
    def __init__(self, num_blocks, to_dim, to_len, from_dim, latent_dim, num_heads):
        super().__init__()
        self.decoder = nn.ModuleList(
            [
                DecoderTransformerBlock(to_dim, to_len, from_dim, latent_dim, num_heads)
                for i in range(num_blocks)
            ]
        )

    def forward(self, X_to, X_from):
        for i in range(len(self.decoder)):
            X_to = self.decoder[i](X_to, X_from)
        return X_to


class Transformer(nn.Module):
    def __init__(
        self, num_blocks, to_dim, to_len, from_dim, from_len, latent_dim, num_heads
    ):
        super().__init__()
        self.encoder = TransformerEncoder(
            num_blocks, to_dim, to_len, latent_dim, num_heads
        )
        self.decoder = TransformerDecoder(
            num_blocks, from_dim, from_len, to_dim, latent_dim, num_heads
        )

    def forward(self, X_to, X_from):
        X_to = self.encoder(X_to)
        X_out = self.decoder(X_from, X_to)
        return X_out

We first create the AttentionBlockTensorDict, the attention block using TensorDictModule and TensorDictSequential.

The wiring operation that connects the modules to each other requires us to indicate which key each of them must read and write. Unlike nn.Sequence, a TensorDictSequential can read/write more than one input/output. Moreover, its components inputs need not be identical to the previous layers outputs, allowing us to code complicated neural architecture.

class AttentionBlockTensorDict(TensorDictSequential):
    def __init__(
        self,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            TensorDictModule(
                TokensToQKV(to_dim, from_dim, latent_dim),
                in_keys=[to_name, from_name],
                out_keys=["Q", "K", "V"],
            ),
            TensorDictModule(
                SplitHeads(num_heads),
                in_keys=["Q", "K", "V"],
                out_keys=["Q", "K", "V"],
            ),
            TensorDictModule(
                Attention(latent_dim, to_dim),
                in_keys=["Q", "K", "V"],
                out_keys=["X_out", "Attn"],
            ),
            TensorDictModule(
                SkipLayerNorm(to_len, to_dim),
                in_keys=[to_name, "X_out"],
                out_keys=[to_name],
            ),
        )

We build the encoder and decoder blocks that will be part of the transformer thanks to TensorDictModule.

class TransformerBlockEncoderTensorDict(TensorDictSequential):
    def __init__(
        self,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            AttentionBlockTensorDict(
                to_name,
                from_name,
                to_dim,
                to_len,
                from_dim,
                latent_dim,
                num_heads,
            ),
            TensorDictModule(
                FFN(to_dim, 4 * to_dim),
                in_keys=[to_name],
                out_keys=["X_out"],
            ),
            TensorDictModule(
                SkipLayerNorm(to_len, to_dim),
                in_keys=[to_name, "X_out"],
                out_keys=[to_name],
            ),
        )


class TransformerBlockDecoderTensorDict(TensorDictSequential):
    def __init__(
        self,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            AttentionBlockTensorDict(
                to_name,
                to_name,
                to_dim,
                to_len,
                to_dim,
                latent_dim,
                num_heads,
            ),
            TransformerBlockEncoderTensorDict(
                to_name,
                from_name,
                to_dim,
                to_len,
                from_dim,
                latent_dim,
                num_heads,
            ),
        )

We create the transformer encoder and decoder.

For an encoder, we just need to take the same tokens for both queries, keys and values.

For a decoder, we now can extract info from X_from into X_to. X_from will map to queries whereas X_from will map to keys and values.

class TransformerEncoderTensorDict(TensorDictSequential):
    def __init__(
        self,
        num_blocks,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            *[
                TransformerBlockEncoderTensorDict(
                    to_name,
                    from_name,
                    to_dim,
                    to_len,
                    from_dim,
                    latent_dim,
                    num_heads,
                )
                for _ in range(num_blocks)
            ]
        )


class TransformerDecoderTensorDict(TensorDictSequential):
    def __init__(
        self,
        num_blocks,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            *[
                TransformerBlockDecoderTensorDict(
                    to_name,
                    from_name,
                    to_dim,
                    to_len,
                    from_dim,
                    latent_dim,
                    num_heads,
                )
                for _ in range(num_blocks)
            ]
        )


class TransformerTensorDict(TensorDictSequential):
    def __init__(
        self,
        num_blocks,
        to_name,
        from_name,
        to_dim,
        to_len,
        from_dim,
        from_len,
        latent_dim,
        num_heads,
    ):
        super().__init__(
            TransformerEncoderTensorDict(
                num_blocks,
                to_name,
                to_name,
                to_dim,
                to_len,
                to_dim,
                latent_dim,
                num_heads,
            ),
            TransformerDecoderTensorDict(
                num_blocks,
                from_name,
                to_name,
                from_dim,
                from_len,
                to_dim,
                latent_dim,
                num_heads,
            ),
        )

We now test our new TransformerTensorDict.

to_dim = 5
from_dim = 6
latent_dim = 10
to_len = 3
from_len = 10
batch_size = 8
num_heads = 2
num_blocks = 6

tokens = TensorDict(
    {
        "X_encode": torch.randn(batch_size, to_len, to_dim),
        "X_decode": torch.randn(batch_size, from_len, from_dim),
    },
    batch_size=[batch_size],
)

transformer = TransformerTensorDict(
    num_blocks,
    "X_encode",
    "X_decode",
    to_dim,
    to_len,
    from_dim,
    from_len,
    latent_dim,
    num_heads,
)

transformer(tokens)
tokens
TensorDict(
    fields={
        Attn: Tensor(shape=torch.Size([8, 2, 10, 3]), device=cpu, dtype=torch.float32, is_shared=False),
        K: Tensor(shape=torch.Size([8, 2, 3, 5]), device=cpu, dtype=torch.float32, is_shared=False),
        Q: Tensor(shape=torch.Size([8, 2, 10, 5]), device=cpu, dtype=torch.float32, is_shared=False),
        V: Tensor(shape=torch.Size([8, 2, 3, 5]), device=cpu, dtype=torch.float32, is_shared=False),
        X_decode: Tensor(shape=torch.Size([8, 10, 6]), device=cpu, dtype=torch.float32, is_shared=False),
        X_encode: Tensor(shape=torch.Size([8, 3, 5]), device=cpu, dtype=torch.float32, is_shared=False),
        X_out: Tensor(shape=torch.Size([8, 10, 6]), device=cpu, dtype=torch.float32, is_shared=False)},
    batch_size=torch.Size([8]),
    device=None,
    is_shared=False)

We’ve achieved to create a transformer with TensorDictModule. This shows that TensorDictModule is a flexible module that can implement complex operarations.

Benchmarking

tdtransformer = TransformerTensorDict(
    num_blocks,
    "X_encode",
    "X_decode",
    to_dim,
    to_len,
    from_dim,
    from_len,
    latent_dim,
    num_heads,
)

Inference Time

import time
t1 = time.time()
tokens = tdtransformer(td_tokens)
t2 = time.time()
print("Execution time:", t2 - t1, "seconds")
Execution time: 0.009623289108276367 seconds
t3 = time.time()
X_out = transformer(X_encode, X_decode)
t4 = time.time()
print("Execution time:", t4 - t3, "seconds")
Execution time: 0.006285429000854492 seconds

We can see on this minimal example that the overhead introduced by TensorDictModule is marginal.

Have fun with TensorDictModule!

Total running time of the script: (0 minutes 10.088 seconds)

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