Enables backend preprocessing to take place outside of the backend interface (#51757)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51757
Enables backend preprocessing to take place outside of the backend interface.
What's new:
* A new definition for backend preprocessing (i.e. BackendPreprocessFunction).
* Registration of the backend's PyTorchBackendInterface interface implementation is augmented to take the BackendPreprocessFunction.
* A new registry is created to handle the BackendPreprocessFunction functions, using the backend's name as key.
* When a BackendPreprocessFunction is used, the PyTorchBackendInterface's "preprocess" method is not added to the LoweredModule. Instead, the BackendPreprocessFunction is called and its output used to set the LoweredModule's __processed_module.
Why?:
These changes are needed to avoid forcing backend preprocessing to be part of the LoweredModule, and in the future be able to eliminate "preprocess" from the PyTorchBackendInterface.
This is important for Mobile use cases where "preprocess" can take the bulk of the compilation process, and thus contain code dependencies that we do not want to bring (or cannot bring) to the Mobile binary.
What didn't change:
* Everything is backwards compatible:
** The existing "preprocess" method in PyTorchBackendInterface is still there.
** When backend registration is done without the BackendPreprocessFunction, as before, things work the same way: "preprocess" is added to LoweredModule, and invoked through the module's instance of the backend interface.
Longer term, the plan is to refactor existing users to move to the new backend registration.
ghstack-source-id: 121190883
Test Plan:
Updated existing tests (test_backend.py) to use the new registration mechanism.
Verified test ran and passed (in my OSS build).
Reviewed By: iseeyuan
Differential Revision: D26261042
fbshipit-source-id: 0dc378acd5f2ab60fcdc01f7373616d1db961e61
