add colab callback function for display, auto download model checkpoints, and colab notebook

[EleaZhong]

• added 2 functions for cfg_sample and clip_sample, which allows calling from colab (ipython display cannot show content when using the "!" magic in notebooks)
• added a callback function for colab displays and showing intermediate results
• added download_model function in diffusion/models/models.py, for QOL
• added a colab notebook demo

[EleaZhong]

fixed merge conflits

[Skylion007]

I was about to make a separate PR for commit 40f4bd2 before I saw this one , I'd recommend merging that atleast as it's free and gives a better download bar for notebooks.