Fix composite compliance testing to check for .item() calls (#81060)
Composite compliance is supposed to check if a composite function
calls .item()
([ref](https://github.com/pytorch/pytorch/blob/39db8b3823b8db82396cb979105a83e5e137a02f/torch/testing/_internal/composite_compliance.py#L135-L138)).
This PR fixes that and adds some more documentation.
Why do we need this check? The original motivations are that Tensor subclasses
may not support .item calls (e.g. vmap and ProxyTensor).
There is no way for these subclasses to meaningfully override the .item() calls
in composite functions that exist inside the PyTorch framework without raising
an error* so we should aim to rewrite composite operations to not call .item().
*We're open to other solutions, this is just the one we decided on when we
wrote composite compliance testing and these tests help us keep track of the
failing functionality.
Test Plan:
- wait for tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81060
Approved by: https://github.com/ezyang
