Derive `ElementaryFunctions` conformances for structs. (#25500)
`ElementaryFunctions` derived conformances enable elementary math
functions to work with product spaces formed from
`ElementaryFunctions`-conforming types.
Conform `Differentiable` synthesized associated types to
`ElementaryFunctions` if possible. Similar to existing logic for `VectorProtocol`.
Enables efficient, elegant mathematical optimizers.
