`torch.pow` Add type promotion support and fix issue with __rpow__ (#37098)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37098
### **Cherry-picked from another stack:**
Some code review already occurred here: https://github.com/pytorch/pytorch/pull/32582
### Summary:
Fixes: https://github.com/pytorch/pytorch/issues/32436
The issue caused incorrect handling of dtypes for scalar ** tensor.
e.g. before this change:
```
>>> 5.5 ** torch.ones(5, dtype=torch.int32)
tensor([5, 5, 5, 5, 5], dtype=torch.int32)
```
should return a float tensor.
Also fixes a number of incorrect cases:
* tensors to negative powers were giving incorrect results (1 instead
of 0 or error)
* Behavior wasn't consistent between cuda/cpu
* large_value ** 1 in some cases gave a result not equal
to large_value because of truncation in conversion to double and back.
BC-breaking:
Previously incorrect behavior (in 1.4):
```
>>> a
tensor([1, 1, 1, 1, 1], dtype=torch.int32)
>>> a.pow_(.5)
tensor([1, 1, 1, 1, 1], dtype=torch.int32)
```
After this change:
`RuntimeError: result type Float can't be cast to the desired output type Int`
Test Plan: Imported from OSS
Differential Revision: D21686207
Pulled By: nairbv
fbshipit-source-id: e797e7b195d224fa46404f668bb714e312ea78ac
