Expose `CUDACachingAllocator` `raw_alloc` and `raw_delete` to python (#33860)
Summary:
This PR aims to improve the interoperability with [CuPy](https://github.com/cupy/cupy/pulls).
Instead of having two separate and conflicting memory pools. With this PR, CuPy can directly alloc memory from the PyTorch allocator by means of this proposal https://github.com/cupy/cupy/pull/3126
We would like to gather feedback to know if this approach makes sense for PyTorch, or other alternative designs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33860
Differential Revision: D20212788
Pulled By: ngimel
fbshipit-source-id: bc1e08a66da1992d26021147bf645dc65239581c
