Add Starcoder 2 (#502)

* Add Starcoder2 model and update utils.py

* Refactor model arguments and modules in starcoder2.py

* Refactor FeedForward class to MLP in starcoder2.py

* Fix typo

* pre-commit

* Refactor starcoder2.py: Update model arguments and modules

* Fix LM head and MLP layers

* Rename  input layer norm

* Update bias in linear layers

* Refactor token embeddings in Starcoder2Model

* Rename to standard HF attention layer name

* Add LayerNorm

* Add transposed token embeddings (like in Gemma)

* Refactor MLP and TransformerBlock classes

* Add tie_word_embeddings option to ModelArgs and update Model implementation

* Add conditional check for tying word embeddings in Starcoder2Model

* Fix bias in lm_head linear layer

* Remove unused LayerNorm in stablelm

* Update transformers dependency to use GitHub repository

* fix lm head bug, revert transformer req

* Update RoPE initialization in Attention class

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/README.md

@@ -46,7 +46,7 @@ You can convert models in the Python API with:
 ```python
 from mlx_lm import convert
 
-upload_repo = "mistralai/Mistral-7B-Instruct-v0.1"
+upload_repo = "mlx-community/My-Mistral-7B-v0.1-4bit"
 
 convert("mistralai/Mistral-7B-v0.1", quantize=True, upload_repo=upload_repo)
 ```