Add optimized op_linear (#5243)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5243

If we happen to be running without a delegate, directly implementing linear is much more efficient than permute_copy_out (materialize a transpose) followed by matmul.
ghstack-source-id: 241918986
exported-using-ghexport

Reviewed By: kimishpatel

Differential Revision: D62154007

fbshipit-source-id: 7b764cf9de616729541f081a51384ba8e18e72f5