Deprecate LightningDistributed and keep broadcast in ddp/ddpSpawn directly

[four4fish]

Proposed refactoring or deprecation

LightningDistributed() class only used by ddp and ddpSpawn, and only have one broadcast function for torch collectives. It's unnecessary to have.
Also, we have to set rank and device in set up steps. If subclass extend DDP or DDPSpawn, and overridden function where LightningDistributed.rank and device setted, could cause silent failures.

2. Now the src is not respected in torch broadcast

Motivation

Simplify the code structure and reduce the possibilities for silent failure

Pitch

Deprecate LightningDistributed. There is only one function

    def broadcast(self, obj: Any, group=_group.WORLD):
        # always wrap into a list so it can be broadcasted.
        obj = [obj]

        if self.rank != 0:
            obj = [None] * len(obj)

        broadcast_object_list(obj, 0, group=group or _group.WORLD)

        return obj[0]

Move to ddp and ddpSpawn

   def broadcast(self, obj: object, src: int = 0) -> object:
        if not distributed_available():
            raise RuntimeError("DDP is not initialized and torch.distributed is not available, can not broadcast object")
        obj = [obj]
        if self.global_rank != 0:
            obj = [None] * len(obj)
        broadcast_object_list(obj, src, group=_group.WORLD)
        return obj[0]

Additional context

Related to #7534

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[awaelchli]

The chosen sprint is in the past, updating it.

[awaelchli]

I'm ok with this change. It is reasonable now that the majority of methods have disappeared from LightningDistributed, and after our acclerator rework, there is no longer a need for this standalone class. And it will be replaced by the collective plugin in that sense.