[CrashFix] Make the dst tensor contiguous when copying from metal
Summary: Somehow the destination tensor becomes incontiguous when copying from Metal. We need to call `.contiguous()` explicitly. See the crash log - https://www.internalfb.com/intern/logview/details/facebook_ios_crashes/1d865405fbc1a45f9517470906c9ec08/
Test Plan:
- verify the crash
- Sandcastle CIs
Reviewed By: dreiss
Differential Revision: D25502884
fbshipit-source-id: 46ee720bf6b6658e51cb56a4e4c16ce121eeabc7
