[torchgen] Let native function declaration generation logic take a callable (#90780)
Retry of #90590, which is a retry of #89594. Original PR reverted due to internal breakage.
This PR fixes the breakage by adding a default value to the new argument.
This PR allows `get_native_function_declarations` API to take a function as argument. This function should take `NativeFunction` as input and emit code for native function declaration. By default it is `dest.compute_native_function_declaration`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90780
Approved by: https://github.com/ezyang
