Make torch.conj() a composite function and return self for real tensors (#43270)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43270
`torch.conj` is a very commonly used operator for complex tensors, but it's mathematically a no op for real tensors. Switching to tensorflow gradients for complex tensors (as discussed in #41857) would involve adding `torch.conj()` to the backward definitions for a lot of operators. In order to preserve autograd performance for real tensors and maintain numpy compatibility for `torch.conj`, this PR updates `torch.conj()` which behaves the same for complex tensors but performs a view/returns `self` tensor for tensors of non-complex dtypes. The documentation states that the returned tensor for a real input shouldn't be mutated. We could perhaps return an immutable tensor for this case in future when that functionality is available (zdevito ezyang ).
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D23460493
Pulled By: anjali411
fbshipit-source-id: 3b3bf0af55423b77ff2d0e29f5d2c160291ae3d9
