Drop Python 3.9 support

[eginhard]

• Same as for Coqpit. There are already just very few Python 3.9 installs for coqui-tts and dropping it simplifies the type hints here a lot.
• Add type hints for the main public interface
• Add a consistency check for the optimizer attributes in the config
• Use Pytorch @inference_mode() decorator instead of changing global state with torch.set_grad_enabled(False)