Move MONAI Generative into core

[marksgraham]

As discussed in the core meeting, MONAI Generative is going to be integrated into the core MONAI repository. The plan is to do this in stages:

1. Port the main parts of the generative code (utils, networks, metrics, losses, inferers, engines, and associated tests). This might lead to some redundancy, but we can focus on the port for now and aim to reduce redundancies further on.
2. Once these features are available in dev, we can move the tutorials to the tutorial repo - this should be easy to do, mostly involving updates imports.
3. We can then move the model zoo to the model zoo repo.

I can start working on stage 1 soon. Any thoughts/comments?

@ericspod @Warvito @wyli

Progress

• [x] Losses (PR link)
• [x] Metrics
• [x] Utils
• [x] Networks
• [x] DIffusion schedulers
• [x] Inferers
• [ ] Engines