Add API for listing functions overridable by __torch_function__ (#33791)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/33182
This adds private API functions that developers of types that implement `__torch_function__` can use to ensure full coverage of the subset of the PyTorch API that can be overrided.
I've refactored some of the code in the tests into a new `torch._overrides.get_overridable_functions` function. I've also changed `TENSOR_LIKE_TORCH_OVERRIDES` into `torch._overrides.get_testing_overrides` and `IGNORED_TORCH_FUNCTIONS` into `torch._overrides.get_ignored_functions`. Making these two static global variables in the tests into functions should allow rewriting their implementation to construct their return values instead of just statically defining the return value as is done here. Currently that is blocked on not being able to inspect function signatures of compiled kernels in PyTorch (see https://github.com/pytorch/pytorch/issues/28233). See the docs I've added for usage examples of these new functions. I also refactored the existing override tests to make use of these new functions, which should be a good forcing function to make sure they're kept up-to-date.
Finally, while working on this I discovered that `TestTorchFunctionOverrides.test_mean` and `TestTorchFunctionOverrides.test_mm` weren't ever being run because they were getting clobbered by the other dynamically generated override tests. I fixed that by renaming the tests and then fixing the actual test code. I've verified that all the subclassing semantics is correct and that the updated test answers are correct. I'm happy to put the fixes to the existing tests in as a separate pull request if that would be easier to review.
ping cpuhrsch since the feature request originally came from them.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33791
Differential Revision: D20195053
Pulled By: cpuhrsch
fbshipit-source-id: 1585f4e405f5223932b410eae03a288dc8eb627e
