Add method to retrieve used chat template (huggingface#32032)

encapsulate chat template logic

src/transformers/tokenization_utils_base.py

@@ -1772,54 +1772,7 @@ def apply_chat_template(
         if tokenizer_kwargs is None:
             tokenizer_kwargs = {}
 
-        using_default_template = False
-
-        # First, handle the cases when the model has a dict of multiple templates
-        if isinstance(self.chat_template, dict) or (
-            self.chat_template is None and isinstance(self.default_chat_template, dict)
-        ):
-            if self.chat_template is not None:
-                template_dict = self.chat_template
-                using_default_dict = False
-            else:
-                template_dict = self.default_chat_template
-                using_default_dict = True
-            if chat_template is not None and chat_template in template_dict:
-                # The user can pass the name of a template to the chat template argument instead of an entire template
-                chat_template = template_dict[chat_template]
-                if using_default_dict:
-                    using_default_template = True
-            elif chat_template is None:
-                if tools is not None and "tool_use" in template_dict:
-                    chat_template = template_dict["tool_use"]
-                elif "default" in template_dict:
-                    chat_template = template_dict["default"]
-                else:
-                    raise ValueError(
-                        "This model has multiple chat templates with no default specified! Please either pass a chat "
-                        "template or the name of the template you wish to use to the `chat_template` argument. Available "
-                        f"template names are {sorted(template_dict.keys())}."
-                    )
-                if using_default_dict:
-                    using_default_template = True
-
-        elif chat_template is None:
-            # These are the cases when the model has a single template
-            # priority: `chat_template` argument > `tokenizer.chat_template` > `tokenizer.default_chat_template
-            if self.chat_template is not None:
-                chat_template = self.chat_template
-            else:
-                chat_template = self.default_chat_template
-                using_default_template = True
-
-        if using_default_template:
-            logger.warning_once(
-                "No chat template is set for this tokenizer, falling back to a default class-level template. This is "
-                "very error-prone, because models are often trained with templates different from the class default! "
-                "Default chat templates are a legacy feature and will be removed in Transformers v4.43, at which "
-                "point any code depending on them will stop working. We recommend setting a valid chat template before "
-                "then to ensure that this model continues working without issues."
-            )
+        chat_template = self.get_chat_template(chat_template, tools)
 
         if return_assistant_tokens_mask and not re.search(r"\{\%-?\s*generation\s*-?\%\}", chat_template):
             logger.warning_once(
@@ -2012,6 +1965,77 @@ def activate_tracker(self, rendered_blocks: list[int], generation_indices: list[
         jinja_env.globals["raise_exception"] = raise_exception
         return jinja_env.from_string(chat_template)
 
+    def get_chat_template(self, chat_template: Optional[str] = None, tools: Optional[List[Dict]] = None) -> str:
+        """
+        Retrieve the chat template string used for tokenizing chat messages. This template is used
+        internally by the `apply_chat_template` method and can also be used externally to retrieve the model's chat
+        template for better generation tracking.
+
+        Args:
+            chat_template (`str`, *optional*):
+                A Jinja template or the name of a template to use for this conversion.
+                It is usually not necessary to pass anything to this argument,
+                as the model's template will be used by default.
+            tools (`List[Dict]`, *optional*):
+                A list of tools (callable functions) that will be accessible to the model. If the template does not
+                support function calling, this argument will have no effect. Each tool should be passed as a JSON Schema,
+                giving the name, description and argument types for the tool. See our
+                [chat templating guide](https://huggingface.co/docs/transformers/main/en/chat_templating#automated-function-conversion-for-tool-use)
+                for more information.
+
+        Returns:
+            `str`: The chat template string.
+        """
+        using_default_template = False
+        # First, handle the cases when the model has a dict of multiple templates
+        if isinstance(self.chat_template, dict) or (
+            self.chat_template is None and isinstance(self.default_chat_template, dict)
+        ):
+            if self.chat_template is not None:
+                template_dict = self.chat_template
+                using_default_dict = False
+            else:
+                template_dict = self.default_chat_template
+                using_default_dict = True
+            if chat_template is not None and chat_template in template_dict:
+                # The user can pass the name of a template to the chat template argument instead of an entire template
+                chat_template = template_dict[chat_template]
+                if using_default_dict:
+                    using_default_template = True
+            elif chat_template is None:
+                if tools is not None and "tool_use" in template_dict:
+                    chat_template = template_dict["tool_use"]
+                elif "default" in template_dict:
+                    chat_template = template_dict["default"]
+                else:
+                    raise ValueError(
+                        "This model has multiple chat templates with no default specified! Please either pass a chat "
+                        "template or the name of the template you wish to use to the `chat_template` argument. Available "
+                        f"template names are {sorted(template_dict.keys())}."
+                    )
+                if using_default_dict:
+                    using_default_template = True
+
+        elif chat_template is None:
+            # These are the cases when the model has a single template
+            # priority: `chat_template` argument > `tokenizer.chat_template` > `tokenizer.default_chat_template
+            if self.chat_template is not None:
+                chat_template = self.chat_template
+            else:
+                chat_template = self.default_chat_template
+                using_default_template = True
+
+        if using_default_template:
+            logger.warning_once(
+                "No chat template is set for this tokenizer, falling back to a default class-level template. This is "
+                "very error-prone, because models are often trained with templates different from the class default! "
+                "Default chat templates are a legacy feature and will be removed in Transformers v4.43, at which "
+                "point any code depending on them will stop working. We recommend setting a valid chat template before "
+                "then to ensure that this model continues working without issues."
+            )
+
+        return chat_template
+
     @property
     def default_chat_template(self):
         """