[ONNX] add supplement for standardOps low precision cast (#60731) (#61561)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61561
address Gary reply and add supplement of https://github.com/pytorch/pytorch/pull/53813.
- add more details for LowPrecisionCastNodeForStandardOps to make it more comprehensible.
- remove unuse gemm test
Test Plan: Imported from OSS
Reviewed By: nikithamalgifb
Differential Revision: D29767991
Pulled By: SplitInfinity
fbshipit-source-id: d00032e13699f5b02fc619e64aa8fdd39f3a66b8
Co-authored-by: hwangdeyu <dejack953@outlook.com>
