[fx2trt] save and load TRTModule for OSS (#65958)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65958
zhxchen17 added `pickle` pybind for trt engine which allows us to save and load a nn.Module with trt engine in fbcode. This diff though is explicitly ser/des engine in __set_state__` and `__get_state__` so that in OSS people can also save and load TRTModule directly.
Test Plan: buck test mode/dev-nosan caffe2/torch/fb/fx2trt:test_fx2trt
Reviewed By: wushirong
Differential Revision: D31309429
fbshipit-source-id: 9068e2ae6375ed0e1bb55b0e9d582b8d9c049dbf
