Remove TrainingTypePlugin.post_dispatch in favor of teardown

[awaelchli]

What does this PR do?

Part of #10896 and follow-up to #10885

In #10896 the spawn logic will get redesigned, making the concept of dispatch and post dispatch obsolete. What used to be called post_dispatch does no longer need to be differentiated from teardown. Changes in this PR:

• Trainer calls accelerator.teardown()
• Accelerator will call training_type_plugin.teardown()
• The current TTP will call super().teardown() in it's inherited methods
• The ParallelPlugin will call teardown also on the cluster environment

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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)

cc @Borda @justusschock @awaelchli @akihironitta @tchaton @kaushikb11

[tchaton]

LGTM !

[kaushikb11]

LGTM!

[four4fish]

Awesome! Thank you for address this! How about pre_dispatch, could we deprecate that as well?

[awaelchli]

Yes, my idea was to merge it with TTP.setup() method directly after #10896 lands. Given the simplifications that #10896 offers, I think that's the next logical step.