Rename DispatchKey Dense/Sparse/etc to DenseFunctionality/SparseFunctionality, use original name for alias

[ezyang]

🚀 The feature, motivation and pitch

This means you could register a kernel as "Sparse" and it would apply to all sparse backends, instead of having to register SparseCPU, SparseCUDA one by one as we do today

Alternatives

No response

Additional context

No response

cc @ezyang @bhosmer @smessmer @ljk53 @bdhirsh

[ezyang]

cc @bdhirsh