Add proper jinja2 error (#35533)

* Cleanup jinja2 imports

* Raise a proper error if Jinja is missing

* make fixup

src/transformers/utils/chat_template_utils.py

@@ -362,6 +362,11 @@ def _render_with_assistant_indices(
 
 @lru_cache
 def _compile_jinja_template(chat_template):
+    if not is_jinja_available():
+        raise ImportError(
+            "apply_chat_template requires jinja2 to be installed. Please install it using `pip install jinja2`."
+        )
+
     class AssistantTracker(Extension):
         # This extension is used to track the indices of assistant-generated tokens in the rendered chat
         tags = {"generation"}