LoRA: Remove unnecessary model type judgments (#388)

* LoRA: Remove unnecessary model type judgments

1. Supported models are already checked in the load_model function in utils, no need to repeat the check in lora
2. The checks in lora are not synchronized with those in utils

* LoRA: add LoRA supported models in mlx_lm utils

llms/mlx_lm/utils.py

@@ -25,6 +25,9 @@ MODEL_MAPPING = {
     "qwen": qwen,
     "plamo": plamo,
 }
+LORA_SUPPORTED_MODELS = [
+    llama.Model, mixtral.Model, phi2.Model, stablelm_epoch.Model
+]
 MAX_FILE_SIZE_GB = 5
 
 linear_class_predicate = (