Fixing minor issues in llama2 7b repro (#2926)

Summary:
Pull Request resolved: #2926

Fixing issues we've seen in #2907 and #2805

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Reviewed By: iseeyuan, cccclai

Differential Revision: D55893925

fbshipit-source-id: c6e0264d868cb487faf02f95ff1bd223cbcc97ac
(cherry picked from commit 6db9d72)

examples/models/llama2/README.md

@@ -61,10 +61,17 @@ You can export and run the original Llama2 7B model.
 
 1. Llama2 pretrained parameters can be downloaded from [Meta's official website](https://ai.meta.com/resources/models-and-libraries/llama-downloads/) or from [Hugging Face](https://huggingface.co/meta-llama/Llama-2-7b).
 
-2. Export model and generate `.pte` file:
+2. Edit `params.json` file. Replace `"vocab_size": -1` with `"vocab_size": 32000`. This is a short-term workaround.
+
+3. Export model and generate `.pte` file:
     ```
     python -m examples.models.llama2.export_llama --checkpoint <checkpoint.pth> --params <params.json> -kv --use_sdpa_with_kv_cache -X -qmode 8da4w --group_size 128 -d fp32
     ```
+4. Create tokenizer.bin.
+
+    ```
+    python -m examples.models.llama2.tokenizer.tokenizer -t tokenizer.model -o tokenizer.bin
+    ```
 
 ### Option B: Download and export stories110M model