Enable use of transformers.MPT model.

Using that keyword there allows the open_flamingo factory to successfully return a model with the `trust_remote_code =False`.
The returned model is of type transformers.models.mpt.modeling_mpt.MptForCausalLM.
May be useful to other people.

config.json

@@ -29,7 +29,7 @@
   "low_precision_layernorm": true,
   "max_seq_len": 2048,
   "mlp_ratio": 4,
-  "model_type": "mosaic_gpt",
+  "model_type": "mpt",
   "n_heads": 16,
   "n_layers": 24,
   "no_bias": true,