fix: add support for wide characters when building index of dataset files

[mattmazzola]

Context

When reading a dataset for the first time metaseq will create an "index" file that will store the byte offset in the .jsonl file where item N begins

Issue

This calculation cur += len(line) was previously assuming the "length" of the read line was the offset of "next" item.
In the case that the line contains wide Unicode characters, this calculation is incorrect. len(line) will return 1 instead of the total bytes of the character.

Example

An example where len gives us the wrong number with a normal string that contains wide-unicode char
The correct length is given if we use a byte-string

>>> "\u2205"
'∅'
>>> len("\u2205")
1
>>> len(b"\u2205")
6

Solution

Instead of keeping a running counter this PR simplifies the implementation by querying the current position of the file reader after every line is read. This approach properly accounts for the bytes and not the characters.

Credit goes to @tupini07 for diagnosing and fixing.

One of the important bug fixes from #726

Testing steps

We noticed this bug in our repo when testing datasets containing these wide characters and have been using it in our pipeline for few weeks now.

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[suchenzang]

Thanks for catching!