fix llm as judge warnings

[NathanHB]

[NathanHB]

Now loads openai lazily. the dep is no linger required when using lighteval

[clefourrier]

You could add a warning/error when loading metrics at the task level, to check there the deps if needed.

[NathanHB]

You could add a warning/error when loading metrics at the task level, to check there the deps if needed.

you mean like I did in task.py to check that the token is set ?

[clefourrier]

And for both of these, I mean using the same checks as for all the other optional deps :) (using new functions defined in utils)

[clefourrier]

@NathanHB I added the import testing I was thinking about - I'll let you review and if it works for you you can merge