Add DDPSpawnPlugin.spawn()

[awaelchli]

What does this PR do?

Part of #9987

Adds a spawn() method for creating new processes. This new API can be independently called (from outside the trainer) and is specific to the Spawn family of plugins (TPUSpawn, ShardedSpawn, etc.). This PR adds the base method and replaces calls to mp.spawn elsewhere with this new method.

Does your PR introduce any breaking changes? If yes, please list them.

Yes, process idx was removed from the method DDPSpawnPlugin.new_process. The TPUSpawnPlugin will be updated separately in a new PR.

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[kaushikb11]

LGTM!

[kaushikb11]

@awaelchli There are some failing tests, regarding passing parameters to the new_process method