[PyTorch Mobile] Export Operator List from Mobile CompilationUnit instead of from TorchScript Model (#49385)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49385
Currently, the API to export operator lists accepts a `torch::jit::Module` object, and spits out an operator list. The operator list is practically used only for mobile. This is not ideal because the set of root operators may change by the time the model is subsequently optmized and exported for mobile.
What we need to to instead is glean the list of operators from the mobile model itself (`bytecode.pkl` specifically), and expose that instead.
Also updated the logic in `converter`.
### Before this change:
1. Get operator List from Torch Script Model
2. Convert to bytecode mobile model
### After this change:
1. Convert to bytecode mobile model
2. Use this converted mobile model to get the list of operators for each method on the model
ghstack-source-id: 118796752
Test Plan:
Added a unit test in `test_lite_interpreter.cpp` to ensure that all model referenced operators show up in the exported operator list. Also make `test_lite_interpreter.cpp` runnable from `xplat/caffe2/BUCK` since this is where the production code will be built from.
Verified that the list of operators produced before and after this change for an example model (segmentation) are the same.
{P147863234}
Also verified that the operator lists for BI-Xray model is different (we have been having problems with missing operators for this one): {P154903132}
Reviewed By: iseeyuan
Differential Revision: D24690094
fbshipit-source-id: 0426a6ef90456a811010cfe337c415882ae2deff
