Let DDPSpawnPlugin.spawn return a result from rank 0

[awaelchli]

What does this PR do?

Part of #9987

Short term motivation: Lite has currently the ability to spawn its run() method by calling the DDPSpawnPlugin.spawn() method. However there is currently no mechanism to transfer a result back to the main process. We solve this problem by allowing the run() function to return a "pickleable" result from rank 0. This gets transferred to the main process through a queue, which gets added in this PR.

Long term motivation: #10059

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[tchaton]

LGMT !

[SeanNaren]

is there a test for this somewhere?

[awaelchli]

@SeanNaren there there are two tests in Lite #9987 and I just improved one more to check also the rank.