Remove DDPSpawnStrategy.get_mp_spawn_kwargs in favor of launchers

[awaelchli]

What does this PR do?

Fixes #10895
Follow up to #11643

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @kaushikb11 @awaelchli @akihironitta @rohitgr7

[kaushikb11]

LGTM! Just start_method needs to be removed from _XLASpawnLauncher's constructor.

Also, loving the Launcher Strategy interface.