Fix precision issues in CPU remainder (#38293)
Summary:
Together with https://github.com/pytorch/pytorch/issues/37758, this fixes https://github.com/pytorch/pytorch/issues/37743 and fixes https://github.com/pytorch/pytorch/issues/24861.
This follows the CUDA fix in https://github.com/pytorch/pytorch/issues/37758, vectorised using a `blendv` to replace the if conditionals.
Most of the complication is from `remainder` supporting `at::Half` where `fmod` doesn't. I've now got `fmod` working on `Vec256<at::Half>` as well as enabling half dispatch for `fmod` so it matches `remainder`.
I also added `fmod` support to `Vec256<at::BFloat16>` before realising that `remainder` doesn't support `BFloat16` anyway. I could also enable `BFloat16` if that's desirable. If not, I don't think `Vec256<BFloat16>` should be missing `fmod` anyway.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38293
Differential Revision: D21539801
Pulled By: ezyang
fbshipit-source-id: abac6a3ed2076932adc459174cd3d8d510f3e1d5
