add training mode to mobile::Module (#42880)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42880
Enable switching between and checking for training and eval mode for torch::jit::mobile::Module using train(), eval(), and is_training(), like exists for torch::jit::Module.
Test Plan: Imported from OSS
Reviewed By: iseeyuan
Differential Revision: D23063006
Pulled By: ann-ss
fbshipit-source-id: b79002148c46146b6e961cbef8aaf738bbd53cb2
