Raise a proper error if Jinja is missing

Author: Rocketknight1

src/transformers/utils/chat_template_utils.py

@@ -28,6 +28,8 @@
 
 if is_jinja_available():
     import jinja2
+    from jinja2.ext import Extension
+    from jinja2.sandbox import ImmutableSandboxedEnvironment
 else:
     jinja2 = None
 
@@ -360,11 +362,14 @@ def _render_with_assistant_indices(
 
 @lru_cache
 def _compile_jinja_template(chat_template):
-    class AssistantTracker(jinja2.ext.Extension):
+    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"}
 
-        def __init__(self, environment: jinja2.sandbox.ImmutableSandboxedEnvironment):
+        def __init__(self, environment: ImmutableSandboxedEnvironment):
             # The class is only initiated by jinja.
             super().__init__(environment)
             environment.extend(activate_tracker=self.activate_tracker)
@@ -418,7 +423,7 @@ def tojson(x, ensure_ascii=False, indent=None, separators=None, sort_keys=False)
     def strftime_now(format):
         return datetime.now().strftime(format)
 
-    jinja_env = jinja2.sandbox.ImmutableSandboxedEnvironment(
+    jinja_env = ImmutableSandboxedEnvironment(
         trim_blocks=True, lstrip_blocks=True, extensions=[AssistantTracker, jinja2.ext.loopcontrols]
     )
     jinja_env.filters["tojson"] = tojson