feat(evaluator): calculate average of metric result across all dataset

[serious-gist]

Issue

Currently mmocr does not returns the avergae of all metric results in MultiDatasetEvaluator

Solution

Adding an method which calculates the average of all the metric results.

How to tackle this PR.

Just go through the changes made and test cases that are added.

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[serious-gist]

@Harold-lkk I have made changes as per my requirement, please review and comment. Let me know if there is anything else.

[OpenMMLab-Assistant-004]

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