[ONNX] Update behavior for `register_custom_op_symbolic` (#85636)
Update `register_custom_op_symbolic`'s behavior to _only register the symbolic function at a single version_. This is more aligned with the semantics of the API signature.
As a result of this change, opset 7 and opset 8 implementations are now seen as fallback when the opset_version >= 9. Previously any ops internally registered to opset < 9 are not discoverable by an export version target >= 9. Updated the test to reflect this change.
The implication of this change is that users will need to register a symbolic function to the exact version when they want to override an existing symbolic. They are not impacted if (1) an implementation does not existing for the op, or (2) they are already registering to the exact version for export.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85636
Approved by: https://github.com/BowenBao
