[feature] Support multi-gpu test multiple datasets separately

[Mactarvish]

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[zhouzaida]

Hi @Mactarvish , thank you for your contribution. I think this feature can be implemented in a new Evaluator MultiDatasetEvaluator rather than in the Metric. We can discuss this first in this PR.

[Mactarvish]

Hi @Mactarvish , thank you for your contribution. I think this feature can be implemented in a new Evaluator MultiDatasetEvaluator rather than in the Metric. We can discuss this first in this PR.

Thanks for your suggestion. I've been trying this way.