[Relax] Batch norm correctness on eval mode

[hugolatendresse]

Batch_norm is a different operator in training and eval. The previous interface defaulted to the training mode and required changing an ingested pytorch program itself to use the eval mode. This is sub-ideal, especially since torch.export explicitely communicates whether batch_norm should be in training or eval in a given torch program.

This PR automates the selection of training/eval mode in the exported program translator, and achieves correctness for eval mode.

Future TODO: there is something wrong with batch_norm on training mode. It does not pass a correctness test when taken straight from the main branch (there's an issue with tensor dimensions). I added a note to address later as training mode is probably not high priority.