Add ESM-2 protein language model

[vincentamato]

I implemented Meta AI's ESM-2 protein language model in MLX! I've included:

• Complete ESM-2 implementation (8M to 15B parameter models)
• HuggingFace model conversion script
• Benchmarking scripts which show ~34% speedup over PyTorch MPS on my M4 MacBook Pro
• Example notebooks for using the model
• Tests validating equivalence with original implementation

The key to this is in the notebooks that guide you through using the model.