Accepts BatchEncoding in LengthGroupedSampler
Author: tma15
Pull request: yes

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Expands `LengthGroupedSampler` to accept `BatchEncoding`-based `Dataset`  by auto inference of lengths of them as well as `dict`-based `Dataset`.
Because `BatchEncoding` can be seen as a special type of dictionary in Python, it is useful to be accepted  by `LengthGroupedSampler`


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