BatchNorm fixes for JAX and PyTorch workloads

[priyakasimbeg]

Fixes to BatchNorm behavior in JAX and PyTorch; mainly decouple update batch norm statistics from using the running statistics.

Changes for PyTorch from @adefazio's #783

From pull/783:
There are some subtle issues with how BatchNorm is handled in the PyTorch version of the code. Currently, workload.model_fn has an update_batch_norm parameter, which in theory should allow the submission to control whether the batch-norm statistics are updated during a forward pass. The issues are the following:

• The update_batch_norm_fn function stores the old momentum parameter for each batchnorm layer in a momentum_backup variable, so it can be restored later, before zeroing the parameter. However, if it is called with update_batch_norm=False twice in a row, it overwrites the momentum_backup with 0 on the second call, so momentum then remains zero for the remainder of training.
• In PyTorch's bultin BatchNorm, 0 indicates that the momentum buffer shouldn't be updated. This is the opposite of how EMA momentum is usually done (i.e. in Adam), where 1 would indicate that it shouldn't be updated, and 0 means it's set to the latest value at every step. The custom BatchNorm modules used in the two librispeech workloads follows this second, more standard convention instead. However, the update_batch_norm_fn sets the momentum to zero for all three layer types, resulting in incorrect behavior for the librispeech workloads.
• The update_batch_norm_fn sets the BN layers to eval mode. This doesn't make sense as it prevents the use-case where you use batch-computed statistics (train mode) without also updating the running statistics. The BN layers can bet set to eval mode separately by passing in ForwardPassMode.EVAL to the forward pass, so removing this .eval() call doesn't prevent the submission from using eval mode during a forward pass.
  This PR changes switch the custom BN code to follow the BN convention so that momentum=0 doesn't update the running buffers. It also fixes the issues in the update_batch_norm_fn function mentioned above.

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