Add warning if behavior name not found in trainer config (#4204)

Co-authored-by: Ervin T. <ervin@unity3d.com>
Co-authored-by: Chris Elion <chris.elion@unity3d.com>

ml-agents/mlagents/trainers/trainer_util.py

         self.ghost_controller = GhostController()
 
     def generate(self, brain_name: str) -> Trainer:
+        if brain_name not in self.trainer_config.keys():
+            logger.warning(
+                f"Behavior name {brain_name} does not match any behaviors specified in the trainer configuration file:"
+                f"{sorted(self.trainer_config.keys())}"
+            )
         return initialize_trainer(
             self.trainer_config[brain_name],
             brain_name,