revert len(datamodule) PR

[ninginthecloud]

What does this PR do?

Revert PR #9895

See the discussion in Issue #5965
overriding _len_ of datamodule could bring a lot of downstream problems. Let's revert this change (PR #9895 ) and discuss a better way to summarize datamodule.

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[ninginthecloud]

cc: @tchaton @ananthsub @carmocca

[tchaton]

LGTM !