[ONNX] remove outdated ImplicitCastType QA in onnx.rst (#81268)
Extend work from: https://github.com/pytorch/pytorch/pull/80596
This PR removes outdated QA of ImplicitCastType , as the coverage is greatly increased with the introduction of onnx shape inference and scalar type analysis.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81268
Approved by: https://github.com/justinchuby, https://github.com/BowenBao
