Fix evaluation batch size validation to use num_generations_eval when specified

Problem:

The validation check in GRPOConfig.__post_init__ (lines 830-836) (

)
always validates the evaluation batch size divisibility against num_generations, even when num_generations_eval is explicitly set.

Solution:

Modified the check to use num_generations_eval when it's set, falling back to num_generations only when num_generations_eval is None.

Impact:

• Fixes incorrect validation errors when users set different generation counts for training vs evaluation
• Allows proper use of the num_generations_eval parameter

Changes:

• Added logic to determine the correct generation count for evaluation validation
• Updated error message to be more accurate

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Was this discussed/approved via a GitHub issue? Please add a link
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?