Flatten the Strategy inheritance

[four4fish]

Proposed refactor

Flatten the Strategy inheritance: Part of #10416

Motivation

Reduce coupling between strategies, reduce unintentional overrides/inheritance and avoid silent failures eg:

1. DeepSpeed inherited configure_ddp() from DDP, which is unnecessary and error-prone.
2. DDP and FSDP have totally different distributed behavior, but with current inheritance, adding a DDP strategy method will automatically enable this method for FSDP

Pitch

Tasks list:

• FSDP inherit ParallelStrategy instead of DDP
• DeepSpeed inherit ParallelStrategy instead of DDP
• [RFC] DDPShard inherit ParallelStrategy instead of DDP
• [RFC] DDPShardSpawn inherit ParallelStrategy instead of DDPSpawn
• [RFC] TPUSpawn inherit ParallelStrategy instead of DDPSpawn

Additional context

The downside of this task will be code duplications. Align DDP/DDPSpawn precess group creation #11643 and Collective refactor #9414 will reduce the duplication.

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cc @justusschock @awaelchli @akihironitta @rohitgr7

[awaelchli]

Let's first get process launchers #11643 done so the ddp launch logic does not get duplicated into every strategy? cc @rohitgr7