Redo of "Add API for listing functions overridable by __torch_function__" (#34240)
Summary:
This is a redo of https://github.com/pytorch/pytorch/pull/33791, which was reverted because it introduced a flaky test. The test was flaky and only flaky on Python3.5 because of dict order randomization.
I've fixed the issue with tests clobbering each other in b539fec and removed the override tests for `torch.nn.functional.tanh` and `torch.nn.functional.sigmoid`, which are deprecated and shouldn't be overridable in e0d7402. I also verified that no more test clobbering is happening.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34240
Differential Revision: D20252442
Pulled By: cpuhrsch
fbshipit-source-id: 069568e342a41c90e1dc76cbf85ba4aed47f24be
