Add a batch_first arg to Transformer / MHA modules (#55285)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/25100 #43112
EDIT: pardon my inexperience since this is my first PR here, that I did not realize the doc should not have any trailing white spaces, and `[E712] comparison to False should be 'if cond is False:' or 'if not cond:'`, now both fixed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55285
Reviewed By: mruberry
Differential Revision: D27765694
Pulled By: jbschlosser
fbshipit-source-id: c34774fa065d67c0ac130de20a54e66e608bdbf4
