Llama 3.2 rope scale factor is hardcoded to 8

[mergennachin]

🐛 Describe the bug

Between Llama 3.1 and 3.2, rope_scale factor had increased from 8 to 32.

https://huggingface.co/.../Llama-3.2-3B.../discussions/1
https://huggingface.co/.../f6dcb8adea08358576f9a68d351035...

But our code is still hard-coded to https://github.com/pytorch/executorch/blob/main/examples/models/llama/rope.py#L20

Versions

N/A

[mergennachin]

#7272