Update on "Dtype selective build for optimized ops"

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).

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

[ghstack-poisoned]

Author: lucylq