GitLab

* [fix] add and use get_process_group_cached

- This commit makes FSDP avoid making too many process groups by default
- Extra process group is bad for GPU memory and init time

* add changelog

* lint

* note on speed

* add better assert output

test seems to be flaky:
https://app.circleci.com/pipelines/github/facebookresearch/fairscale/2957/workflows/383c9f9f-f1a5-461c-8c41-e2e28ece037b/jobs/26783/steps



* update test reference memory values

- With cached process groups, the memory is reduced as reported by
pytorch as well (due to bucket buffer memory for the reduction buffer)
- The effect on memory is actually more on the SMI memory, which is not
reported by pytorch and checked by this test.

* Update fairscale/nn/data_parallel/fully_sharded_data_parallel.py

* Update fairscale/nn/data_parallel/fully_sharded_data_parallel.py

* Update CHANGELOG.md

* Update fairscale/utils/parallel.py

* Update fairscale/utils/parallel.py

* Update fairscale/utils/parallel.py

* Update fairscale/utils/parallel.py

* improved changelog

* better handling of underscores in the md file
Co-authored-by: Min Xu <min.xu@acm.org>

NCCL all_to_all is now supported in PyTorch (since v1.8.0)

Fixes: #548

* tentatively fixing the cpu version of circleci jobs, now pipe tests are the last ones standing
* fixing oss backcompat, trying to fix rpc in old pytorch also
* fixing the file based init in torch 1.5

* file based dist init
* nicer handling of broken world sizes vs. number of available GPUs, do not break but warn out

Adds support for:
* Reused layers (e.g. for weight sharing)
* Lazily-constructed layers
* Single-process control via PipeRPCWrapper
* PipelineStyle.AsyncScheudle, which lays the foundation for asynchronous pipeline work by introducing an event loop for each rank/worker to process either activations or gradients as they arrive

Also added examples for multi-process and PipeRPCWrapper

The expert annotation is used by clip_grads and DDP.

Currently only implemented for a single process and expert.