Proposed refactor

Now we have {strategy_name}+{parameter} registries, but not {strategy_name} for most strategies. For example “ddp_find_unused_parameters_false” is registries but not “ddp”. The accelerator_connector uses both strategy_registry and enums to find str to strategy matching.
part of #10416

Motivation

Unify strategy registry behavior, remove enums and enable accelerator_connector logic simplification

Pitch

Step 1: define distributed_backend for every strategy. Like in DDPStrategy
https://github.com/PyTorchLightning/pytorch-lightning/blob/f41d1e5e5ebb7040a39d137695e818cada9a9234/pytorch_lightning/strategies/ddp.py#L83

Step 2: Registry strategy with the strategy.distributed_backend

    @classmethod
    def register_strategies(cls, strategy_registry: Dict) -> None:
        strategy_registry.register(
            cls.distributed_backend,
            cls,
            description=f"{cls.__class__.__name__} Strategy",
        )

Step 3: part of the #11449
Simplify accelerator_connector logic

Additional context

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cc @justusschock @awaelchli @akihironitta @rohitgr7