Standalone nanotron config

[hynky1999]

What does this implement/fix? Explain your changes.

This PR moves the lighteval config to lighteval codebase.

• Enforces the lighteval_config_path as the only way to read the lighteval config. The nanotron part is ignore, this way the breaking changes won't be as breaking.
• Some typing corrections
  Comments

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• Haven't tested yet as I don't want to rebase on nanotron fix PR so waiting for that to be merged

[NathanHB]

I took a look, tested and fixed a few bumps, it should work tell me what you thikn !

[hynky1999]

@NathanHB Thanks for fixing the problems.
I made small nits to the config (outpid_dir type to str, as that's what it's in cli interface).

Two more things:

• I changed the nanotron model to take batch_size from override_bs (as do accelerate models), I still think it's very confusing, but leaving it as it is
• I noticed that batch detection for nanotron doesn't work. Created an issue for that. I think it's low prio but let's keep track of it [BUG] Nanotron batch detection doesn't work #286