NeMo - replace named tuples with dataclasses in dataloader returns.

[krishnacpuvvada]

    No action item below, but need to consider long term.

I wonder if we should start moving away from named tuples, switching to data classes instead. NamedTuple somehow interfere with pickling which might become necessary soon with PT 2.0 (maybe)

Originally posted by @titu1994 in #5356 (comment)