Fix sample dataset indexing

[zzachw]

          We should probably add a check. But, it's mostly so we don't break compatibility with legacy code like split_by_visit() in https://github.com/sunlabuiuc/PyHealth/blob/master/pyhealth/datasets/splitter.py

Originally posted by @jhnwu3 in #335 (comment)

[ram-from-tvl]

Hi @zzachw
Could you please assign this issue..?
I would like to work on this.
Thank you!

[jhnwu3]

Hey happy to always receive help @ram-from-tvl ! Thanks!