Patching Batch Norm

What’s happening?

Batch Norm requires in-place updates to running_mean and running_var of the same size as the input. Functorch does not support inplace update to a regular tensor that takes in a batched tensor (i.e. regular.add_(batched) is not allowed). So when vmaping over a batch of inputs to a single module, we end up with this error

How to fix

All of these options assume that you don’t need running stats. If you’re using a module this means that it’s assumed you won’t use batch norm in evalution mode. If you have a use case that involves running batch norm with vmap in evaluation mode, please file an issue

Option 1: Change the BatchNorm

If you’ve built the module yourself, you can change the module to not use running stats. In other words, anywhere that there’s a BatchNorm module, set the track_running_stats flag to be False

BatchNorm2d(64, track_running_stats=False)

Option 2: torchvision parameter

Some torchvision models, like resnet and regnet, can take in a norm_layer parameter. These are often defaulted to be BatchNorm2d if they’ve been defaulted. Instead you can set it to BatchNorm that doesn’t use running stats

import torchvision
from functools import partial
torchvision.models.resnet18(norm_layer=partial(BatchNorm2d, track_running_stats=False))

Option 3: functorch’s patching

functorch has added some functionality to allow for quick, in-place patching of the module. If you have a net that you want to change, you can run replace_all_batch_norm_modules_ to update the module in-place to not use running stats

from functorch.experimental import replace_all_batch_norm_modules_
replace_all_batch_norm_modules_(net)