[fx2trt] [Prep for release] Add type hints to converters and separate main files (#69458)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69458
1. Added type hints to acc ops converters.
2. Put some of the class/logic in fx2trt.py to some separated files. (input_tensor_spec.py, trt_module.py, converter_registry.py).
3. Added import in `__init__.py` so that user can just call `from torch.fx.experimental.fx2trt import xxx` instead of `experimental.fx2trt.fx2trt`.
Test Plan: CI
Reviewed By: wushirong
Differential Revision: D32884637
fbshipit-source-id: e3e1e597edb9a08b47b4595bd371f570f2f3c9b6
