[Examples] Test and update READMEs on multi-GPU support

[sayakpaul]

We keep getting issues where community members want to know how to do multi-GPU training with our training examples. An example: #2981.

I think we should test the following examples on a priority (as for others, multi-GPU is probably overkill) and ensure their respective READMEs have a short section on multi-GPU support (as suggested by @patrickvonplaten here):

• Text-to-image
• ControlNet
• InstructPix2Pix (being introduced here)

DreamBooth and Textual Inversion won't probably benefit from multi-GPU setup that much, so okay for me to not focus on them at the moment.

I will test text-to-image as well as InstructPix2Pix. @williamberman or @yiyixuxu could do it for ControlNet?

[pure-rgb]

multi-GPU, does it refer to data parallel or model parallel, or both?

[sayakpaul]

DDP.

[williamberman]

Yeah would be fun!