Update group_norm.py for TensorFlow/Keras compatibility

[mohammadnabia]

Updated the import statements in group_norm.py to ensure compatibility with the latest versions of TensorFlow/Keras. Adjusted the references to get_custom_objects, Layer, and InputSpec to reflect their new locations in recent updates. Reason for Changes These changes are intended to maintain the functionality of the codebase with the most current versions of TensorFlow/Keras. I hope this helps keep the project up to date and running smoothly.

Thank you for considering this contribution!