fix(server): Handle loading from local files for MPT (#534)

This PR allows the MPT model to be loaded from local files. Without this
change, an exception will be thrown by `hf_hub_download` function if
`model_id` is a local path.

server/text_generation_server/models/mpt.py

 import torch
 import torch.distributed
 
+from pathlib import Path
 from typing import Optional, Type
 from opentelemetry import trace
 from transformers import AutoTokenizer, PretrainedConfig, PreTrainedTokenizerBase
@@ -60,7 +61,12 @@ class MPTSharded(CausalLM):
         )
         tokenizer.pad_token = tokenizer.eos_token
 
-        filename = hf_hub_download(model_id, revision=revision, filename="config.json")
+        # If model_id is a local path, load the file directly
+        local_path = Path(model_id, "config.json")
+        if local_path.exists():
+            filename = str(local_path.resolve())
+        else:
+            filename = hf_hub_download(model_id, revision=revision, filename="config.json")
         with open(filename, "r") as f:
             config = json.load(f)
         config = PretrainedConfig(**config)