Char level ppl

[sdtblck]

Sorry, my autoformatter went ham.
Probably not the worst thing.

Relevant changes are:

• adds a char counter here: https://github.com/EleutherAI/gpt-neox/compare/char_level_ppl?expand=1#diff-b6ee2c1db780b46787ab3a576020f9a12695f3f5cb38ba8badcd5501960e5d22R440
• adds the option to turn on char level ppl here: https://github.com/EleutherAI/gpt-neox/compare/char_level_ppl?expand=1#diff-fefa29324180c69866ca857d29fb03443ef143b9fee5aa5217cd5f5e5ae2b82cR892
• In eval, wraps the data iterator in a char counter that counts the characters and tokens as we iterate through it (needed to do it this way because we don't have access to the deepspeed internals when doing pipe parallel), then uses this info to calculate the char level ppl here: https://github.com/EleutherAI/gpt-neox/compare/char_level_ppl?expand=1#diff-e2b248f8c422a601bcb0b7d93f96c1dff070f2694737e2b69f1def64ab9c1844R662

[StellaAthena]

I have resolved the conflicts with main. @sweinbach can you confirm that this doesn't introduce bugs for whatever reason?

[sdtblck]

@StellaAthena I tested again, works fine. Can you reapprove?