[quant][pt2e] Allow users to override train/eval behavior (#119091)
Summary: This commit adds a util for PT2E quantization users
to call `model.train()` and `model.eval()` without error.
Instead, these will automatically call the equivalent
`move_exported_model_to_train/eval` for the user, which only
switch behavior for special ops like dropout and batchnorm.
This enables users to onboard to the PT2E flow more easily.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_allow_exported_model_train_eval
Reviewers: jerryzh168, tugsbayasgalan, zhxchen17
Subscribers: jerryzh168, tugsbayasgalan, zhxchen17, supriyar
Differential Revision: [D53426636](https://our.internmc.facebook.com/intern/diff/D53426636)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119091
Approved by: https://github.com/jerryzh168, https://github.com/tugsbayasgalan, https://github.com/zhxchen17
