[quant][pt2e] Add `model_is_exported` util function (#119726)
Summary: This commit adds the `model_is_exported` util function
for users to be able to easily tell what APIs to call to move
their models between train and eval modes. This has the
additional advantage of hiding the implementation of how we
detect a model is exported, in case the metadata format changes
in the future.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_model_is_exported
Differential Revision: [D53812972](https://our.internmc.facebook.com/intern/diff/D53812972)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119726
Approved by: https://github.com/tugsbayasgalan, https://github.com/albanD
