[ONNX] Expand `_cast_` symbolic functions (#87666)
The `_cast_` family of symbolic functions has been created from a template function. Even though it saved some lines, it very much obscured the intention of the code. Since the list doesn't really change and the `_cast_` family are IIRC deprecated, it is safe for us to expand the templates and make the code more readable.
This PR also removes any direct calls to `_cast_` functions to maintain a consistent pattern of directly creating `Cast` nodes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87666
Approved by: https://github.com/BowenBao
