Create "Inferer" class for Diffusion Models specifically

To make the concept of "Inferer" classes more clear, diffusion model inferers, which all follow a specific signature and are required both to train diffusion models and run inference from them, should be separated from the other inferers.

Currently, they all sit in the same class: inferers.py.

Describe the solution you'd like
A separate inferer class for diffusion models.

Additional context
This was part of the requirements for porting Generative Models to Core.

Considerations

• Modify diffusion models class
• Modify every call to inferers
• Solve twin issue in MONAI tutorials: https://github.com/Project-MONAI/tutorials/issues/2027
• Solve twin issue in MONAI model zoo: https://github.com/Project-MONAI/model-zoo/issues/765