Update the model type to make it compatible with mlx-lm's model mapping.

Once the model type is updated, it should be ready to be ported into mlx-lm and able to be lora fine-tuned with gate.

config.json

@@ -16,7 +16,7 @@
   "img_processor": null,
   "initializer_range": 0.02,
   "layer_norm_epsilon": 1e-05,
-  "model_type": "phi-msft",
+  "model_type": "phixtral",
   "n_embd": 2560,
   "n_head": 32,
   "n_head_kv": null,