Make CUDACachingAllocator::recordStream() a no-op on null ptrs (#20658)
Summary:
Fixes #20651
Communication collectives in `torch.distributed` call `CUDACachingAllocator::recordStream()` on input and output tensors to prevent their memory blocks being freed too early. `CUDACachingAllocator` uses tensor's data pointer to track memory blocks, which does not accept null pointers. However, empty tensor's `storage().data()` might be null. In this case, as there is no associated memory block for the empty tensor, it should be fine to make `recordStream()` a no-op.
Tests only cover `broadcast` empty tensors for GLOO backend, because GLOO does not support empty inputs (facebookincubator/gloo/issues/179). It can be addressed in either `ProcessGroupGloo` or GLOO itself. Will add more tests when that gap is filled.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20658
Differential Revision: D15399371
Pulled By: mrshenli
fbshipit-source-id: d29ebd1c72fddae49531f32695f81b89e42e5a4d
