[tests] Add Context-parallel CI tests
Author: kashif
Pull request: yes

# What does this PR do?

Adds two context parallel tests for the CI

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_41860). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Looks fantastic, Kashif! Thank you for adding the missing tests. Now I don't need to worry about breaking your CP integration while integrating ALST/UlyssesSP.

oh, one request, since we will now have more than one context parallel, you might want to rename the file+class to include `torch` in it - via https://github.com/huggingface/transformers/pull/41832 and https://github.com/huggingface/accelerate/pull/3817 we will now have `pc.cp_backend = {torch|deepspeed}` `torch` being the default.

Thanks a lot. I will check on our multi-gpu CI runners too.

would it be possible to exercise the rest of the torch/fsdp PC config here:
```
parallelism_config:
  parallelism_config_dp_replicate_size: 1
  parallelism_config_dp_shard_size: 1
  parallelism_config_tp_size: 1
  parallelism_config_cp_size: 2

  --------------
  parallelism_config_cp_comm_strategy: alltoall
```

that is to add all config values for the Torch PC plugin, like `cp_comm_strategy`? it also helps with tests-as-documentation/example



Kashif, I'd suggest dropping the basic test, since it gets repeated almost identically in the 2nd test - will save $$ and time. 

@kashif, I took your test as a foundation for mine and further worked on it, while also making it self-contained w/o needing to include in the already bloated repo more files. 

https://github.com/huggingface/transformers/pull/41832/files#diff-bfd2f0924ca5096a9cf7bff5929081cf6552b70df8bb40632d3b11273c9554af

Please feel free to re-use, with the note that I made some tweaks to `testing_utils.py` that it relies on - your version of my lib is quite outdated, I have made quite a few changes over the recent years. If you want to sync back the latest version is here https://github.com/stas00/ml-engineering/blob/master/testing/testing_utils.py

@stas00 I've made the script in the self-contained style and incorporated your suggestions

@ydshieh we should ideally run this only on the multi-gpu setup, is that possible?

Looks great, Kashif. 

@SunMarc moved to fsdp folder



> @ydshieh we should ideally run this only on the multi-gpu setup, is that possible?

Yes, we have multi-gpu runners (A10). I will check how it goes with that.


@kashif The test is run and pass in our multi-gpu A10 runner, great!

amazing thank you @ydshieh