Update base for Update on "add eval for attention sink"

This PR adds the function to evaluate the model's perplexity when AttentionSink is enabled.

This is mostly copied from https://github.com/mit-han-lab/streaming-llm/blob/main/examples/eval_long_ppl.py which is used by the AttentionSink paper to evaluate the model's perplexity when AttentionSink is enabled.

Differential Revision: [D66474732](https://our.internmc.facebook.com/intern/diff/D66474732/)

Perplexity measured for llama 3.2 1B and 1B_Instruct model up to 40k tokens with AttentionSink enabled:

<img width="966" alt="Screenshot 2024-11-25 at 2 46 04 PM" src="https://github.com/user-attachments/assets/ba7118f9-b5d7-4de8-b1fa-7d2ba0646515">


[ghstack-poisoned]

Author: helunwencser