Adding inference endpoints models

[clefourrier]

Inference endpoints models are these ones: https://huggingface.co/docs/huggingface_hub/v0.20.3/en/package_reference/inference_endpoints
Not to be confused with TGI models (which need a local deployment)

This PR:

• uses Requests instead of passing tuples (which are more error prones) in the Datasets
• introduces an Abstract Model class which defines the minimum functions we need to have in a model for it to be lighteval compatible
• cleans up the BaseModel code
• introduces inference endpoints models