move has_torch_function to C++, and make a special case object_has_torch_function (#48965)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48965
This PR pulls `__torch_function__` checking entirely into C++, and adds a special `object_has_torch_function` method for ops which only have one arg as this lets us skip tuple construction and unpacking. We can now also do away with the Python side fast bailout for `Tensor` (e.g. `if any(type(t) is not Tensor for t in tensors) and has_torch_function(tensors)`) because they're actually slower than checking with the Python C API.
Test Plan: Existing unit tests. Benchmarks are in #48966
Reviewed By: ezyang
Differential Revision: D25590732
Pulled By: robieta
fbshipit-source-id: 6bd74788f06cdd673f3a2db898143d18c577eb42
