Update invariant for collectives in spawn plugins

[ananthsub]

What does this PR do?

This is minor cleanup after #10896 . Processes are spawned first, then the init distributed connection happens. As a result, this PR removes any ambiguity around these collectives: the invariant is that the distributed connection is set up before the trainer calls them.

This removes the possibility of subtle bugs leading when the Trainer calls TrainingTypePlugin.barrier() - if the distributed connection was not initialized, we'd silently pass through, which could lead to synchronization problems later.

Part of #7534

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