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.
 

backends/exllamav2/model.py

@@ -843,12 +843,10 @@ class ExllamaV2Container(BaseModelContainer):
             decode_special_tokens=unwrap(kwargs.get("decode_special_tokens"), True),
         )[0]
 
-    def get_special_tokens(
-        self, add_bos_token: bool = True, ban_eos_token: bool = False
-    ):
+    def get_special_tokens(self):
         return {
-            "bos_token": self.tokenizer.bos_token if add_bos_token else "",
-            "eos_token": self.tokenizer.eos_token if not ban_eos_token else "",
+            "bos_token": self.tokenizer.bos_token,
+            "eos_token": self.tokenizer.eos_token,
             "pad_token": self.tokenizer.pad_token,
             "unk_token": self.tokenizer.unk_token,
         }
@@ -1242,7 +1240,7 @@ class ExllamaV2Container(BaseModelContainer):
         ) and gen_settings.token_repetition_range == -1
 
         stop_conditions = params.stop
-        add_bos_token = params.add_bos_token
+        add_bos_token = unwrap(params.add_bos_token, True)
         ban_eos_token = params.ban_eos_token
 
         # Fetch EOS tokens from generation_config if they exist