Core ATen opset backward & forward compatibility policy

[SS-JIA]

Exactly as @larryliu0820 mentioned, I think in the case of adding operators to the ATen library that can potentially impact the core ATen opset, the core ATen opset should not block additions to the ATen library. In my view, the core ATen opset is a separate system that “tracks” the ATen library (or more specifically, the operators used in PyTorch programs) so it is the responsibility of the maintainers of the core ATen opset to respond to such changes in the ATen library.