[PyTorch] Avoid registering ops into dispatcher in lightweight dispatch
Summary:
This change adds the following logic:
If lightweight dispatch is enabled, do not generate `TORCH_LIBARAY` API calls for operator schema and implementations, since these operators will be registered into JIT op registry.
`skip_dispatcher_op_registration` is an existing argument to `gen.py`. With that set, `RegisterDispatchKey.cpp` will not generate `m.def` and `m.impl` for each native function. This logic will be removed once we find a better way to skip op registration into dispatcher.
Test Plan: Rely on unit tests for lightweight dispatch.
Differential Revision: D34634300
fbshipit-source-id: 691289f6b7b044fed3927fabd249cae89e8eaef9
