fix: remove custom rope

[jbrodriguez]

the create_cos_sin_theta went missing, however it's no longer needed, check #465

this removes the custom class and adds the base param to the default RoPE instantation

[jbrodriguez]

i tested it and it's now working for me

~/tmp/mlx-examples/llms/mixtral main*
❯ python mixtral.py --model-path mlx_model
[INFO] Loading model from disk.
[INFO] Starting generation...
In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.

Douglas Adams, The Hitchhiker's Guide to the Galaxy

## 1.1 Introduction

The Universe is a complex and fascinating place. It is a place of beauty and wonder, but also of mystery and danger. It is a place of great order and great chaos. It is a place of great harmony and great discord. It is a place of great

~/tmp/mlx-examples/llms/mixtral main* 20s

[angeloskath]

Thanks @jbrodriguez for the fix.