Add the __torch_function__ API override mechanism (#30730)
Summary:
This is a re-do of https://github.com/pytorch/pytorch/issues/27064, which was reverted (https://github.com/pytorch/pytorch/commit/b8792c0438f4292aa813c36207f75eebcbb77a45). This was landed at the same time as other work that added new operators to the `torch` namespace so the check for whether the `torch` namespace is exhaustively checked for overridability was triggering test failures.
I've temporarily disabled that check and added an explanatory comment that the check will be re-enabled in a future PR that will be merged during a time when the commit velocity on PyTorch is lower.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30730
Differential Revision: D18813270
Pulled By: ezyang
fbshipit-source-id: 70477c4656dca8fea6e7bc59259555041fcfbf68
