API: Ignore add_bos_token in chat completions

When fetching special tokens from the model, don't factor in the
add_bos_token and ban_eos_token parameters as switches.

In addition, change the internal handling of add_bos_token to an optional
boolean. This allows us to fallback to the model when selecting whether
or not to add the BOS token, especially for chat completions.

Signed-off-by: kingbri <8082010+kingbri1@users.noreply.github.com>

backends/base_model_container.py

@@ -123,7 +123,7 @@ class BaseModelContainer(abc.ABC):
         pass
 
     @abc.abstractmethod
-    def get_special_tokens(self, **kwargs) -> Dict[str, Any]:
+    def get_special_tokens(self) -> Dict[str, Any]:
         """
         Gets special tokens used by the model/tokenizer.