Missed one CHECK_NOTNULL in #82032's find-replace (#84720)
Building master fails with the following:
```
pytorch/caffe2/contrib/nccl/cuda_nccl_gpu.cc:180:51: error: 'CHECK_NOTNULL' was not declared in this scope; did you mean 'TORCH_CHECK_NOTNULL'?
180 | CUDA_ENFORCE(cudaStreamWaitEvent(CHECK_NOTNULL(ex.stream), event, 0));
```
Seems like #82032 just missed one find-replace. cc @wconstab
Not sure why this wouldn't have been caught elsewhere.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84720
Approved by: https://github.com/wconstab
