[WIP] Add IFEval

[nikil-ravi]

This PR adds IFEval. So far,I've tested baseline evaluation with Qwen 3 1.7B so far on an H100, and this runs in just under 15 minutes.

[max-andr]

thanks! we need to discuss internally whether we want to add IFEval to our "official" suite of benchmarks. but i personally like IFEval, since IFEval scores can be improved a lot by doing smarter post-training.