Simplify op copy (#711)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/711

The fact that `src` must broadcast to `in` implies that the result of broadcasting `in` and `src` has shape equal to `in`. Therefore `out` must have the same shape than `in`. Hence, we can avoid calling `get_broadcast_target_size` and simply resize `out` to `in.sizes()`
ghstack-source-id: 203404905
exported-using-ghexport

Reviewed By: SS-JIA

Differential Revision: D49979519

fbshipit-source-id: 5b532c098165451817bc25067ba10ff68f6f0a23