support local model config file (#1058)

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Support local config file to avoid unexpected `discard_names`, which
causes #1057.

In the case of launching local mode without `model.safetensors` file,
the original code will result `discard_names = []` when
`hf_hub_download` throws an connection error.
```python
# server/text_generation_server/cli.py
    try:
        import transformers
        import json
    
    
        config_filename = hf_hub_download(model_id, revision=revision, filename="config.json")
        with open(config_filename, "r") as f:
            config = json.load(f)
        architecture = config["architectures"][0]
    
        class_ = getattr(transformers, architecture)
    
        # Name for this varible depends on transformers version.
        discard_names = getattr(class_, "_tied_weights_keys", [])
        discard_names.extend(getattr(class_, "_keys_to_ignore_on_load_missing", []))
    
    except Exception as e:
        discard_names = []
```

The expected `_tied_weights_keys` of OPT-1.3b is `["lm_head.weight"]`,
and its tied weight `"model.decoder.embed_tokens.weight"` will be kept
in the safetensors conversion. But the above empty `discard_names` will
lead to `"lm_head.weight"` being kept and
`"model.decoder.embed_tokens.weight"` being discard in the subsequent
method `_remove_duplicate_names`, which causes error #1057.

So add a local mode branch to get the expected `discard_names` like
follows. This modification also applies to other models

```python
# server/text_generation_server/cli.py
        if is_local_model:
            config_filename = os.path.join(model_id, "config.json")
        else:
            config_filename = hf_hub_download(model_id, revision=revision, filename="config.json")
```


In addition, when `_tied_weights_keys` or
`_keys_to_ignore_on_load_missing` is `None`, the above code will also
throw an error unexpectedly. This is fixed in PR #1052


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@Narsil

server/text_generation_server/cli.py

@@ -179,7 +179,10 @@ def download_weights(
             import json
 
 
-            config_filename = hf_hub_download(model_id, revision=revision, filename="config.json")
+            if is_local_model:
+                config_filename = os.path.join(model_id, "config.json")
+            else:
+                config_filename = hf_hub_download(model_id, revision=revision, filename="config.json")
             with open(config_filename, "r") as f:
                 config = json.load(f)
             architecture = config["architectures"][0]