Stop monkeypatching Tensor.backward() on `import functorch` (#85152)
Monkeypatching is bad, we should never be doing it. This PR removes
functorch's monkeypatching on Tensor.backward() by adding it directly to
the implementation of Tensor.backward().
As an alternative, we could have done an `import functorch` and used
`functorch._C.are_transforms_active` directly in
`torch/autograd/__init__.py`. The problem with that is that it runs into a
bunch of circular imports.
NB: https://github.com/pytorch/pytorch/issues/72179 is still on my mind.
I didn't choose to do it right now because:
- This PR doesn't make the situation worse than it already is (no
monkeypatching is better than having the monkeypatch)
- We don't have a design for #72179 yet.
Test Plan:
- tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85152
Approved by: https://github.com/soulitzer
