get_repetition_penalty_reward accepts non-positive ngram_size

Reproduction

ngram_size values of zero or less are accepted when constructing the reward function, but fail later during reward computation.

from trl.rewards import get_repetition_penalty_reward

reward_fn = get_repetition_penalty_reward(ngram_size=0)
reward_fn([[1, 2, 3]])

outputs:

Traceback (most recent call last):
  File "reproduce.py", line 4, in <module>
    reward_fn([[1, 2, 3]])
  File "trl/rewards/other_rewards.py", line 78, in __call__
    scaling = 1 - len(set(ngrams)) / len(ngrams)
                  ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~
ZeroDivisionError: division by zero

Negative values have the same problem because no n-grams are constructed.

Expected behavior: validate ngram_size when constructing the reward function and raise an actionable ValueError unless it is greater than zero. This is consistent with the existing construction-time validation for max_penalty.

System Info

• Platform: macOS-26.6.2-arm64-arm-64bit-Mach-O
• Python version: 3.14.6
• TRL version: 1.13.0.dev0+323efb2
• PyTorch version: 2.13.0
• accelerator(s): cpu
• Transformers version: 5.16.1
• Accelerate version: 1.14.0
• Accelerate config: not found
• Datasets version: 5.0.1
• HF Hub version: 1.29.0
• bitsandbytes version: not installed
• DeepSpeed version: not installed
• Liger-Kernel version: not installed
• PEFT version: not installed
• vLLM version: not installed

Checklist

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• I have included my system information
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