Extend max_model_length to prevent context truncation

[eldarkurtic]

With the existing example command for evaluation, both max_model_length and max_new_tokens are set to the same value of 32768. This produces the following warning:

[ WARNING]: context_size + max_new_tokens=33238 which is greater than self.max_length=32768. Truncating context to 0 tokens. (vllm_model.py:276)

We need to increase the max_model_length to a larger value to prevent truncating the context. I ran some tests and the proposed 38768 seems to be large enough for all three tasks I've tested so far: AIME, MATH-500, and GPQA-Diamond.