Make index_add take a scalar argument alpha (#54176)
Summary:
```
index_add(Tensor self, int dim, Tensor index, Tensor source) -> Tensor
```
now becomes
```
index_add(Tensor self, int dim, Tensor index, Tensor source, Scalar alpha=1) -> Tensor
```
Generally, this sounds useful and harmless, and inside PyTorch, we are already needing this feature in `add_out_dense_sparse_cuda`, see the `SparseCUDATensorMath.cu` change in this PR.
**Test not added yet. Will add if after discussion we believe this is a good idea.**
- [ ] TODO: add test
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54176
Reviewed By: ngimel
Differential Revision: D27319198
Pulled By: mruberry
fbshipit-source-id: fe43be082d1230c87c5313458213d5252be2ff23
