[PyTorch] Free up 8 bytes per intrusive_ptr_target (#117986)
We don't need 64-bit reference and weak counts. (We also probably don't need a full 32 bits, but we'll deal with that later.)
Differential Revision: [D52851891](https://our.internmc.facebook.com/intern/diff/D52851891/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/117986
Approved by: https://github.com/ezyang
