Unify buffer_size across TorchData DataPipes

[alexanderbattig]

Fixes: #335

As draft, since the tests don't run for me at the moment.

Also, CLA pending at the moment.

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[ejguan]

I updated the summary and PR title. And, I have an early comment that we might want to add a constant DEFAULT_BUFFER_SIZE to torchdata/_constants.py. And, reference all of usage to this place. Then, we can add a function/context manager to update it if users need to change all buffer size at the same time.