Lazy import + refactor Lora layer addition (#426)

* lazy model import in mlx_lm

* change lora loading

* fix olmo lora

* remove a bunch of unused stuff from plamo

* move phixtral to mlx-lm and out of llms/

llms/mlx_lm/models/llama.py

@@ -9,6 +9,7 @@ from .base import BaseModelArgs
 
 @dataclass
 class ModelArgs(BaseModelArgs):
+    model_type: str
     hidden_size: int
     num_hidden_layers: int
     intermediate_size: int
@@ -18,7 +19,6 @@ class ModelArgs(BaseModelArgs):
     num_key_value_heads: int = None
     rope_theta: float = 10000
     rope_traditional: bool = False
-    model_type: str = None
     rope_scaling: Optional[Dict[str, Union[float, str]]] = None
 
     def __post_init__(self):
@@ -190,6 +190,7 @@ class LlamaModel(nn.Module):
 class Model(nn.Module):
     def __init__(self, args: ModelArgs):
         super().__init__()
+        self.model_type = args.model_type
         self.model = LlamaModel(args)
         self.lm_head = nn.Linear(args.hidden_size, args.vocab_size, bias=False)