fix: tokenizer config should use local model path when possible (#1518)

This PR fixes the issue with loading a local tokenizer config.
Previously the default functionality would look in the current working
directory. Now if a local model path is specified we will check that
directory for the tokenizer_config.

## Examples of valid commands

uses tokenizer_config from hub
```
text-generation-launcher --model-id HuggingFaceH4/zephyr-7b-beta
```

use tokenizer_config from local model path
```
text-generation-launcher \
  --model-id ~/.cache/huggingface/hub/models--HuggingFaceH4--zephyr-7b-beta/snapshots/dc24cabd13eacd3ae3a5fe574bd645483a335a4a/
```

use specific tokenizer_config file
```
 text-generation-launcher \
  --model-id ~/.cache/huggingface/hub/models--HuggingFaceH4--zephyr-7b-beta/snapshots/dc24cabd13eacd3ae3a5fe574bd645483a335a4a/ \
  --tokenizer-config-path ~/.cache/huggingface/hub/models--HuggingFaceH4--zephyr-7b-beta/snapshots/dc24cabd13eacd3ae3a5fe574bd645483a335a4a/tokenizer_config.json


```

---------

Co-authored-by: Nicolas Patry <patry.nicolas@protonmail.com>

router/src/lib.rs

@@ -37,7 +37,7 @@ pub struct HubTokenizerConfig {
 }
 
 impl HubTokenizerConfig {
-    pub fn from_file(filename: &str) -> Self {
+    pub fn from_file(filename: &std::path::Path) -> Self {
         let content = std::fs::read_to_string(filename).unwrap();
         serde_json::from_str(&content).unwrap_or_default()
     }

router/src/main.rs

@@ -154,12 +154,6 @@ async fn main() -> Result<(), RouterError> {
     let local_path = Path::new(&tokenizer_name);
     let local_model = local_path.exists() && local_path.is_dir();
 
-    // Load tokenizer config
-    // This will be used to format the chat template
-    let local_tokenizer_config_path =
-        tokenizer_config_path.unwrap_or("tokenizer_config.json".to_string());
-    let local_tokenizer_config = Path::new(&local_tokenizer_config_path).exists();
-
     // Shared API builder initialization
     let api_builder = || {
         let mut builder = ApiBuilder::new()
@@ -230,24 +224,35 @@ async fn main() -> Result<(), RouterError> {
     };
 
     // Load tokenizer config if found locally, or check if we can get it from the API if needed
-    let tokenizer_config = if local_tokenizer_config {
+    let tokenizer_config = if let Some(path) = tokenizer_config_path {
+        tracing::info!("Using local tokenizer config from user specified path");
+        HubTokenizerConfig::from_file(&std::path::PathBuf::from(path))
+    } else if local_model {
         tracing::info!("Using local tokenizer config");
-        HubTokenizerConfig::from_file(&local_tokenizer_config_path)
-    } else if let Some(api) = api {
-        tracing::info!("Using the Hugging Face API to retrieve tokenizer config");
-        get_tokenizer_config(&api.repo(Repo::with_revision(
-            tokenizer_name.to_string(),
-            RepoType::Model,
-            revision.unwrap_or_else(|| "main".to_string()),
-        )))
-        .await
-        .unwrap_or_else(|| {
-            tracing::warn!("Could not retrieve tokenizer config from the Hugging Face hub.");
-            HubTokenizerConfig::default()
-        })
+        HubTokenizerConfig::from_file(&local_path.join("tokenizer_config.json"))
     } else {
-        tracing::warn!("Could not find tokenizer config locally and no revision specified");
-        HubTokenizerConfig::default()
+        match api {
+            Some(api) => {
+                tracing::info!("Using the Hugging Face API to retrieve tokenizer config");
+                let repo = Repo::with_revision(
+                    tokenizer_name.to_string(),
+                    RepoType::Model,
+                    revision.unwrap_or("main".to_string()),
+                );
+                get_tokenizer_config(&api.repo(repo))
+                    .await
+                    .unwrap_or_else(|| {
+                        tracing::warn!(
+                            "Could not retrieve tokenizer config from the Hugging Face hub."
+                        );
+                        HubTokenizerConfig::default()
+                    })
+            }
+            None => {
+                tracing::warn!("Could not find tokenizer config locally and no API specified");
+                HubTokenizerConfig::default()
+            }
+        }
     };
 
     if tokenizer.is_none() {