[allocator] Introduce the abstract class CUDACachingAllocator (#87251)
This replaces the manual function pointers, making it easier to write
new drop-in allocators.
Note that most allocation goes through the Allocator interface, which
CUDAAllocator inherits from, and this arrangement avoids adding and
additional layer of dispatch along this pathway compared to what existed before.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87251
Approved by: https://github.com/wconstab
