[iOS][CoreML] Check backend availability at runtime. (#65315)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65315
ghstack-source-id: 138703808
Test Plan:
- OSS builds and BUCK builds
- CircleCI
Reviewed By: hanton
Differential Revision: D31048011
fbshipit-source-id: 824a8e32d65de2caf25e41efef2b022ddbb63156
