[torchgen] Refactor static_dispatch to take in source signature (#84384)
Summary: Context: currently `static_dispatch` assumes that given a native function `f`, we always want to map from its `DispatchSignature` to its `CppSignature`. This assumption may not hold true for some use cases, where the source bindings may not come from its `DispatchSignature`. Here I'm changing the argument `sig: DispatcherSignature` to be `sig: Union[CppSignature, DispatcherSignature]`, also removes unused `f`
Test Plan: Rely on added unit test.
Differential Revision: D39192969
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84384
Approved by: https://github.com/iseeyuan
