Add CLI arg generate_until_token to support reasoning and CoT models

[mapmeld]

As noted in #8 and #513 , LightEval expects models to follow a question with an immediate answer, but chain-of-thought and reasoning models (such as DeepSeek) generate many tokens to arrive at a more accurate / thought-out result before answering.

This PR would add --generate-until-token '</think>' as the syntax to support these models.
It must be run with --use-chat-template and a TransformerModel model, or it will raise an Exception.

I have a CoLab notebook running a BigBench task which I didn't run to the end, but I used logger.info to confirm it was generating reasoning text. In a previous test linked in #513 I confirmed this method works on a short task

Notes:

• "generate until token" or "wait until..." is the clearest name I could think of to remind people to use the ending token
• this doesn't look at the tokenizer's chat template string, but it could be helpful to detect the appropriate token
• Should I have do_sample=True when generating the reasoning text? Is that reproducible?
• Is there a way to see logger.debug() when calling lighteval from the command line? I can remove the logging of reasoning text if it isn't helpful
• For better results on my custom task in <think> tags for thinking models #513 , I had to change answers ["A", "B", ...] to ["The answer is A", ...] - thoughts about using the template string to set post-reasoning text and be compatible with more evals?

[EdwardSJ151]

Hi! Is this currently working?

[mapmeld]

@EdwardSJ151 you should be able to run it with the code in this PR. If you run into issues, please comment.
This might help:

• change the logger.debug line to logger.info to confirm that reasoning text is there
• if your answers are "A", "B", "C", you might need to change them to "The answer is A"