Add division overload with rounding_mode selection (#51706)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51706
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50280
As mentioned in gh-43874, this adds a `rounding_mode={'true', 'trunc', 'floor'}`
argument so `torch.div` can be used as a replacement for `floor_divide` during
the transitional period.
I've included dedicated kernels for truncated and floor division which
aren't strictly necessary for float, but do perform significantly better (~2x) than
doing true division followed by a separate rounding kernel.
Note: I introduce new overloads for `aten::div` instead of just adding a default
`rounding_mode` because various JIT passes rely on the exact operator schema.
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D26123271
Pulled By: mruberry
fbshipit-source-id: 51a83717602114597ec9c4d946e35a392eb01d46
