Implement reference counting for shared IPC CUDA tensors (#16854)

Implement reference counting for shared IPC CUDA tensors (#16854)

Summary:
This is to fix #16141 and similar issues.

The idea is to track a reference to every shared CUDA Storage and deallocate memory only after a consumer process deallocates received Storage.

ezyang Done with cleanup. Same (insignificantly better) performance as in file-per-share solution, but handles millions of shared tensors easily. Note [ ] documentation in progress.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16854

Differential Revision: D13994490

Pulled By: VitalyFedyunin

fbshipit-source-id: 565148ec3ac4fafb32d37fde0486b325bed6fbd1