Build MPS delegate with Werror=1 (#1736)

Summary:
Summary of changes:

- Build MPS Delegate with Werror=1 by default (tested on macOS12.0, 13.0 and 14.0)
- Cast the placeholders to FP16 only if initial placeholder is FP32. Other data types remain unchanged.

cc shoumikhin, cccclai, larryliu0820

Pull Request resolved: https://github.com/pytorch/executorch/pull/1736

Reviewed By: cccclai

Differential Revision: D53150094

Pulled By: shoumikhin

fbshipit-source-id: f8f3b4d74ffbf06cfc97a20a115eefd0a44b7ef3