Refactor VariableTypeManual (#30649)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30649
Operators in VariableTypeManual are now no longer registered against the VariableTypeId key, but they are registered as compound ops. See https://github.com/pytorch/pytorch/issues/30102 for background.
This also requires the non-variable codegen to ignore them and requires removal of VariableMethodStubs.cpp.
So, because function_wrapper.py now also needs to know which ops are manual, instead of having a hard-coded list in gen_variable_type.cpp for ops with manual implementation, we now have a `manual_kernel_registration` flag in native_functions.yaml that disables the registration of operator kernels for this operator (the schema is still registered). Then, we manually register the right kernels for the operator.
ghstack-source-id: 95082204
Test Plan: unit tests
Differential Revision: D18778191
fbshipit-source-id: 0af6f9e43ff4fb9800ce19b286dfccd0fd22cc41
