add __torch_function__ API override mechanism (#27064)
Summary:
Closes https://github.com/pytorch/pytorch/issues/24015 (see description of that issue for more details).
For a toy example, see the `DiagonalTensor` and `SubDiagonalTensor` class in test/test_overrides.py.
This PR currently contains:
* tests for `__torch_function__` behavior
* modification to `gen_python_functions` and `parse` function signatures and dispatched to correct overloaded argument.
This feature is inspired by and analogous to NumPy's `__array_function__` protocol ([see NumPy Enhancement Proposal 18](https://numpy.org/neps/nep-0018-array-function-protocol.html#trying-array-function-methods-until-the-right-one-works)).
### Benchmarks:
See Nathan's comment below: https://github.com/pytorch/pytorch/pull/27064#issuecomment-554601189
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27064
Differential Revision: D18645954
Pulled By: ezyang
fbshipit-source-id: 54b5e4344d7afdbcf996bb57191b0bdadc7b1767
