Kevin Tse

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> Can we derive `total_length` from source Datapipe if possible? Updated the implementation to do that with an exception when it cannot infer length from the source.

@nivekt has imported this pull request. If you are a Facebook employee, you can view this diff [on Phabricator](https://www.internalfb.com/diff/D38675266).

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Thanks for the helpful comments. It is simpler than before now!

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