merge interfaces that have an optional scalartype parameter (#21088)
Summary:
This change is backwards incompatible in *C++ only* on mean(), sum(), and prod() interfaces that accepted either of:
```
Tensor sum(IntArrayRef dim, bool keepdim=false) const;
Tensor sum(IntArrayRef dim, ScalarType dtype) const;
```
but now to specify both the dim and dtype will require the keepdim parameter:
```
Tensor sum(IntArrayRef dim, bool keepdim=false, c10::optional<ScalarType> dtype=c10::nullopt) const;
```
[xla ci]
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21088
Reviewed By: ailzhang
Differential Revision: D15944971
Pulled By: nairbv
fbshipit-source-id: 53473c370813d9470b190aa82764d0aea767ed74
