feat: allow null eos and bos tokens in config (#1791)

This PR resolves an issue loading in tokenizer_configs where the eos or
bos token is null as in:
[Qwen/Qwen1.5-72B-Chat](https://huggingface.co/Qwen/Qwen1.5-72B-Chat/blob/main/tokenizer_config.json)

resolves:
https://github.com/huggingface/text-generation-inference/issues/1545 and
related to https://github.com/QwenLM/Qwen1.5/issues/162

router/src/lib.rs

@@ -116,6 +116,7 @@ mod token_serde {
                     ))
                 }
             }
+            Value::Null => Ok(None),
             _ => Err(de::Error::custom("invalid token format")),
         }
     }

router/src/main.rs

@@ -254,7 +254,10 @@ 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 let Some(path) = tokenizer_config_path {
-        tracing::info!("Using local tokenizer config from user specified path");
+        tracing::info!(
+            "Using local tokenizer config from user specified path {}",
+            path
+        );
         HubTokenizerConfig::from_file(&std::path::PathBuf::from(path))
     } else if local_model {
         tracing::info!("Using local tokenizer config");