fix: update max tokens for OpenAI

[Samoed]

While running the benchmark again for all tasks, I ran into an error while evaluating a classification task. Digging deeper, I noticed something weird. OpenAI text embedding models generally have a context length of 8192 (if you go over that, the API throws an error). But it looks like the text-embedding-ada-002 model actually has a context length of 8191. If you push it to 8192 tokens, the API doesn’t throw an error (probably to align with the text-embedding-3-* models), but the model sometimes returns null values instead. I think we should update openai_models.py to fix this. Here’s a script to reproduce the issue, change 8191 to 8192 and see the results.

from openai import OpenAI
import tiktoken
import numpy as np

client = OpenAI()
encoding = tiktoken.get_encoding("cl100k_base")

sn = 'Hello World' * 5000

print(f'Num tokens: {len(encoding.encode(sn))}')

truncated_sentence = encoding.encode(sn)[:8191]
truncated_sentence = encoding.decode(truncated_sentence)

response = client.embeddings.create(
    input=truncated_sentence,
    model="text-embedding-ada-002",
    encoding_format="float",
)

em = np.array(response.data[0].embedding)
print(f'Null values: {np.isnan(em.astype(np.float32)).sum()}')

Originally posted by @HSILA in #1708 (comment)

We had difference num of max_tokens in loader and model meta and that caused this problem

Checklist

• Run tests locally to make sure nothing is broken using make test.
• Run the formatter to format the code using make lint.