Dtype selective build for optimized ops (#10992)

Pull Request resolved: #10878

Add dtype selective build for optimized ops. Follows the same process as portable, where we copy the source files and rebuild the library.

1. Generalize copy genrule for portable/optimized/source/header.
2. Copy optimized source files + headers.
3. Build optimized ops using source files, dependencies, portable header.
4. Add test, confirm that we can run addmul with float dtypes (when we remove, the test fails).
ghstack-source-id: 284862896
@exported-using-ghexport

Differential Revision: [D74688554](https://our.internmc.facebook.com/intern/diff/D74688554/)