Merge duplicated code in DDPSpawn and TPUSpawn plugin

[awaelchli]

What does this PR do?

Part of #10034, #10059

The implementation of the method __recover_child_process_weights in DDPSpawnPlugin and TPUSpawnPlugin are identical and can be merged.

Merge after #10846

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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)