add MPTBlock to _no_split_modules

the blocks.py is a separate file vs [standard transformers modeling scripts](https://github.com/huggingface/transformers/blob/94056b57beb4499f4f74d5d88a41e8266cc01778/src/transformers/models/longt5/modeling_longt5.py#L1273) that I am used to, so it took me a bit to find.

modeling_mpt.py

@@ -32,7 +32,7 @@ class MPTPreTrainedModel(PreTrainedModel):
     config_class = MPTConfig
     base_model_prefix = "model"
     supports_gradient_checkpointing = True
-    _no_split_modules = []
+    _no_split_modules = ["MPTBlock"]
 
 
 class MPTModel(MPTPreTrainedModel):