Add train() / eval() / is_training() to C++ ScriptModule API (#16044)
Summary:
This PR aims to fix https://discuss.pytorch.org/t/how-to-change-a-loaded-model-to-evaluation-mode-in-c/32330, by adding `train()` / `eval()` / `is_training()` to C++ ScriptModule API.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16044
Differential Revision: D13857724
Pulled By: yf225
fbshipit-source-id: 16d3969fb5840ff7e66c7f72e800e6c75db8d2ff
