caffe2: fix PinnedCPUAllocator cudaHostRegister() leak (#16340)

caffe2: fix PinnedCPUAllocator cudaHostRegister() leak (#16340)

Summary:
In the NUMA case, PinnedCPUAllocator's allocate() would return a
DataPtr constructed by DefaultCPUAllocator, which would reference
the Default... Delete() rather than the Pinned... Delete(). That
meant Pinned... Delete() would never run, so cudaHostUnregister()
would never be called when regions were freed.

See: https://github.com/pytorch/pytorch/issues/16280

This change adds a 'naked_allocate()' method to the Default allocator
that just returns a pointer to the allocated memory rather than
wrapping it in a DataPtr. Pinned allocator uses that then constructs
a DataPtr with reference to its own Delete().
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16340

Reviewed By: dzhulgakov

Differential Revision: D13843206

Pulled By: ezyang

fbshipit-source-id: 9efb572e5a01b49ef2a4aceeccc13cd0b1066528