Refactoring + fill majority answer script

[Kipok]

This PR has the following changes:

1. Creating new default sft config that does not use sandbox and does not force math accuracy as validation loss
2. Refactor all benchmark-specific settings into a separate file + create new grading/metrics files for benchmark-specific classes
3. Add a script to replace answers with majority answer that we use for gsm8k augmentation
4. New parameter generation_suffix for sft data prep to allow adding eot token that we currently need