Add whitelist for complex backward (#45461)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45461
This PR disables autograd for all C -> C, R -> C functions which are not included in the whitelist `GRADIENT_IMPLEMENTED_FOR_COMPLEX`. In practice, there will be a RuntimeError during forward computation when the outputs are differentiable:
```
>>> x=torch.randn(4, 4, requires_grad=True, dtype=torch.cdouble)
>>> x.pow(3)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: pow does not support automatic differentiation for outputs with complex dtype.
```
The implicit assumption here is that all the C -> R functions have correct backward definitions. So before merging this PR, the following functions must be tested and verified to have correct backward definitions:
`torch.abs` (updated in #39955 ), `torch.angle`, `torch.norm`, `torch.irfft`, `torch.istft`.
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D23998156
Pulled By: anjali411
fbshipit-source-id: 370eb07fe56ac84dd8e2233ef7bf3a3eb8aeb179
