2/n Consolidate collective functions - collective base and subclasses

[four4fish]

What does this PR do?

Implementing proposal here https://docs.google.com/document/d/1e83FcZHHHsTmiUmNpgmPTBYjugZaC3pcZhd4VV9AuIM/edit

Steps:

1. [RFC] create pytorch_lightning/utilities/collective_util.py and move collective related utils from distributed.py to collective_util.py
2. 2/n Consolidate collective functions - collective base and subclasses. (this PR)
3. Integrate with all training_type_plugins (ddp, dp, fullshard and etc) and Accelerators

Fixes #7534

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[codecov]

Codecov Report

Merging #9414 (f293720) into master (b98ce0a) will decrease coverage by 4%.
The diff coverage is 47%.

❗ Current head f293720 differs from pull request most recent head 522ebf9. Consider uploading reports for the commit 522ebf9 to get more accurate results

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[ananthsub]

• for getting something initially working, one PR makes sense, especially to have clear up the API requirements. but once the API requirements are clear, I think this will be easier to review in a stack, especially for the implementations. that'll make it easier so we don't miss anything during the carryover period
• from the gdoc (@yifuwang ) for torch distributed, the process group initialization could be part of the collectives. do you think it makes sense to offer setup/teardown functions on the collective interface that allow for this? this way, the trainer can know if it was the agent which initialized the global process group vs not (cc @kaushikb11 as it potentially relates to Move init_ddp_connection to distributed utilities #9044)

[ananthsub]

• we shouldn't mention early stopping here
• the world size is also needed

[four4fish]

Good catch, how about add world size to be a param in torch_collective, and setup at ddp setup_distributed() and dp go with default 1. Only Torch collective needs this?

[bowangbj]

+1 the cmt looks quite specific.

what does reduce_boolean really mean here? whats the reduce op given list of boolean? probably add a sentence to explain and add a concrete example.

[ananthsub]

I don't think reduce_boolean_decision should be part of the collective interface. This is really higher order functionality for how the Training Type plugin can use the collective to reach consensus across ranks. So I think this should sit at the training type plugin interface instead of here

[awaelchli]

can you add these classes to the project.toml for mypy types?
we want newly added classes and files to be fully type checked

[awaelchli]

I like it very much.

3/n will be the integration?

[four4fish]

I like it very much.

3/n will be the integration?

Yep, will integrate with ttps and accelerators.

[ananthsub]

@four4fish at this point, I don't think offering a new collective base class that unifies all these disparate backends is the right approach. It offers new API surface area in Lightning but can't guarantee that the behavior from the different underlying libraries won't diverge or have different semantics.

IMO this should be addressed at the torch distributed level, especially for XLA usage. In the meantime, Lightning can get by calling the existing utilities as-is inside of the corresponding strategy class.

[carmocca]

@ananthsub are you advocating to drop this proposal entirely?

is there any specific reason for the change of direction?

[four4fish]

@ananthsub are you advocating to drop this proposal entirely?

is there any specific reason for the change of direction?

@carmocca @awaelchli I have synced up with Ananth offline. Not changing direction or drop this plan, we are thinking about switch the implementation order a bit.

1. Unified collective behavior should be provided from pytorch side, now there is effort going on in pytorch to moving to this direction. We will keep you guys updated.
2. I think precision plugin refactor and accelerator refactor in [Main Issue] Accelerator and Plugin refactor #10416 is higher priority towards stable API 1.6.

How about let's have this after the step 2 and 3. (I'm working on the PR for #7324 right now)
What do you guys think? I can also rebase and update this first.

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