Simplify TPUSpawn rank management

[awaelchli]

What does this PR do?

Fixes #10986

The attributes tpu_local_core_rank and tpu_global_core_rank today behave identically to the local_rank and global_rank attributes. They are no longer needed.

It is a breaking change for everyone who was accessing them through the Strategy.

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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 @carmocca @JackCaoG @Liyang90 @gkroiz @justusschock @awaelchli @kaushikb11 @rohitgr7 @akihironitta

[akihironitta]

It is a breaking change for everyone who was accessing them through the Strategy.

@awaelchli Are we removing these attributes without any deprecation? To me, it looks quite inconsistent that these are removed instantly in v1.6 while SingleTPUPlugin will be removed in v1.8.

[awaelchli]

@awaelchli Are we removing these attributes without any deprecation?

Yes. I will add the deprecation if reviewers specifically request it.

To me, it looks quite inconsistent that these are removed instantly in v1.6 while SingleTPUPlugin will be removed in v1.8.

Yes it is inconsistent because not all changes we want to do for 1.6 can be made backward compatible or deprecation-friendly.

[akihironitta]

LGTM!

[awaelchli]

Waiting for TPU CI to be reactivated