pow scalar exponent / base autodiff, fusion (#19324)
Summary:
Fixes: #19253
Fixing pow(Tensor, float) is straightforward.
The breakage for pow(float, Tensor) is a bit more subtle to trigger, and fixing needs `torch.log` (`math.log` didn't work) from the newly merged #19115 (Thanks ngimel for pointing out this has landed.)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19324
Differential Revision: D15003531
Pulled By: ailzhang
fbshipit-source-id: 8b22138fa27a43806b82886fb3a7b557bbb5a865
