Fix bug in grpo reward module import (axolotl-ai-cloud#2571)

Author: dhruvmullick

src/axolotl/core/trainers/grpo/__init__.py

@@ -135,7 +135,9 @@ def get_reward_func(cls, reward_func_fqn: str) -> RewardFunc:
         try:
             # use importlib to dynamically load the reward function from the module
             reward_func_module_name = reward_func_fqn.split(".")[-1]
-            reward_func_module = importlib.import_module(reward_func_fqn.split(".")[-2])
+            reward_func_module = importlib.import_module(
+                ".".join(reward_func_fqn.split(".")[:-1])
+            )
             reward_func = getattr(reward_func_module, reward_func_module_name)
             if not len(inspect.signature(reward_func).parameters) >= 2:
                 raise ValueError(