[custom_ops] make custom_ops.impl work on existing operators (#106076)
The design is that we construct a CustomOp object around the existing
operator and then use it to register things. It is totally OK if the
operator isn't functional (unlike torch._custom_ops.custom_op that can
only construct functional operators).
If the operator already has an implementation from a backend (either via
direct registration to e.g. DispatchKey::CPU, or an indirect
registration like CompositeImplicitAutograd/CompositeExplicitAutograd),
we raise an error.
Test Plan:
- new tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106076
Approved by: https://github.com/soulitzer
ghstack dependencies: #106075
