Allow store to be set for process group init

[ananthsub]

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What does this PR do ?

This change allows the torch.distributed Store to be an attribute of the strategy classes in Nemo. This allows users to customize the store used for the global process group creation before the trainer runs.

By default, the store argument in init_process_group is None (ref) so this PR introduces no functional changes.

init_process_group is not directly accessible or overrideable from the Lightning strategies.

I've copied the code from the superclasses (setup_distributed / setup_environment ) and the util function Lightning relies in order to be able to directly call this function and pass the store in.

TLDR:
All of these changes are so we can go from this:

torch.distributed.init_process_group(self._process_group_backend, rank=global_rank, world_size=world_size)

to this:

torch.distributed.init_process_group(self._process_group_backend, rank=global_rank, world_size=world_size, store=self.store)

The motivation for this PR is to make subsequent PRs integrating Nemo with in-process restart more focused.

Collection: [Note which collection this PR will affect]

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[maanug-nv]

thanks, LGTM.