# functorch¶

## Function Transforms¶

 vmap vmap is the vectorizing map; vmap(func) returns a new function that maps func over some dimension of the inputs. grad grad operator helps computing gradients of func with respect to the input(s) specified by argnums. grad_and_value Returns a function to compute a tuple of the gradient and primal, or forward, computation. vjp Standing for the vector-Jacobian product, returns a tuple containing the results of func applied to primals and a function that, when given cotangents, computes the reverse-mode Jacobian of func with respect to primals times cotangents. jvp Standing for the Jacobian-vector product, returns a tuple containing the output of func(*primals) and the “Jacobian of func evaluated at primals” times tangents. jacrev Computes the Jacobian of func with respect to the arg(s) at index argnum using reverse mode autodiff jacfwd Computes the Jacobian of func with respect to the arg(s) at index argnum using forward-mode autodiff hessian Computes the Hessian of func with respect to the arg(s) at index argnum via a forward-over-reverse strategy.

## Utilities for working with torch.nn.Modules¶

In general, you can transform over a function that calls a torch.nn.Module. For example, the following is an example of computing a jacobian of a function that takes three values and returns three values:

model = torch.nn.Linear(3, 3)

def f(x):
return model(x)

x = torch.randn(3)
jacobian = jacrev(f)(x)
assert jacobian.shape == (3, 3)


However, if you want to do something like compute a jacobian over the parameters of the model, then there needs to be a way to construct a function where the parameters are the inputs to the function. That’s what make_functional() and make_functional_with_buffers() are for: given a torch.nn.Module, these return a new function that accepts parameters and the inputs to the Module’s forward pass.

 make_functional Given a torch.nn.Module, make_functional() extracts the state (params) and returns a functional version of the model, func. make_functional_with_buffers Given a torch.nn.Module, make_functional_with_buffers extracts the state (params and buffers) and returns a functional version of the model func that can be invoked like a function. combine_state_for_ensemble Prepares a list of torch.nn.Modules for ensembling with vmap().

If you’re looking for information on fixing Batch Norm modules, please follow the guidance here