What does this PR do?

Because at the end of the day the trainer will always use a DispatchDataLoader (even in the case of num_gpus == 1), it needs to be able to set the random seed properly for the sampler. The solution is to just always use the SeedableRandomSampler. While our tests (in accelerate)do not fail, they don't account for if the base case is DispatchDataLoader (Trainer tests).

Fixes # (issue)

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@BenjaminBossan