Add reduce variants for `scatter` operation. (#57015)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/56463 #56464
- Add reduce variants for `scatter` in both _native_functions.yaml_ and _TensorAdvancedIndexing.cpp_
- Add `OpInfo` tests and reduce tests in _test_torch.py_
- Fix default reduce argument for `scatter_` in __tensor_docs.py_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57015
Reviewed By: mrshenli
Differential Revision: D28162657
Pulled By: ezyang
fbshipit-source-id: 4d37ed1569ce8560aca1085c9cf5349f11427c4f
