MemPool is a separate pool of memory handled by the caching allocator. This PR adds the option let the caching allocator try to use this pool as a last resort instead of OOMing by associating a use_on_oom bool with each MemPool.
Usage:
Users can optionally specify a ``use_on_oom`` bool (which is False by default) during MemPool creation. If true, then the CUDACachingAllocator will be able to use memory in this pool as a last resort instead of OOMing.
```
pool = torch.cuda.MemPool(allocator, use_on_oom=True)
with torch.cuda.use_mem_pool(pool):
a = torch.randn(40 * 1024 * 1024, dtype=torch.uint8, device="cuda")
del a
# at the memory limit, this will succeed by using pool's memory in order to avoid the oom
b = torch.randn(40 * 1024 * 1024, dtype=torch.uint8, device="cuda")
```
Testing:
```
python test/test_cuda.py -k test_mempool_limited_memory_with_allocator
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/151487
Approved by: https://github.com/eqy, https://github.com/syed-ahmed, https://github.com/ngimel
This PR refactors some ref-counting functionality out of `beginAllocateToPool` and `releasePool`. The ref-counting logic is then used in construction and destruction of `torch.cuda.MemPool`.
The `use_count` variable in the CUDACachingAllocator is essentially a refcount of how many context managers are using the pool. Since we are now lifting up the MemPool abstraction to the user, the MemPool object itself now needs to hold a an extra reference as well.
Part of https://github.com/pytorch/pytorch/issues/124807.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/133600
Approved by: https://github.com/eqy, https://github.com/ezyang
