Fix: update MLX imports for compatibility

[pbatra0]

This PR updates the import statements in utils.py to use the correct module paths for MLX and MLX_LM, ensuring compatibility with the latest versions.

Changes:

• Updates imports for generate_step and stream_generate to use mlx_lm.generate.
• Adds detailed ImportError output for easier debugging.
• No functional changes beyond import path corrections.

These changes resolve import errors and improve maintainability for future updates.