FLUX CFG support

[RyanJDick]

Summary

Add support for Classifier-Free Guidance with FLUX.

• Using CFG doubles the time for the denoising process. Running both the positive and negative conditioning in a single batch is left for future work, because most users are already VRAM-constrained (this would probably be faster at the cost of higher peak VRAM).
• Negative text conditioning is optional and only required if cfg_scale != 1.0
• CFG is skipped if cfg_scale == 1.0 (i.e. no compute overhead in this case)
• cfg_scale_start_step and cfg_scale_end_step can be used to easily control the range of steps that CFG is applied for.
• CFG is a prerequisite for IP-Adapter support.

Example

Positive Caption: Professional photography of a luxury hotel in the Nevada desert
CFG: 1.0

Positive Caption: Professional photography of a luxury hotel in the Nevada desert
Negative Caption: Swimming pool
CFG: 2.0
Same seed

QA Instructions

• Test interactions with ControlNet
• Verify that peak RAM/VRAM utilization has not increased significantly
• Test that CFG is skipped when cfg_scale == 1.0
• Test that negative text conditioning can be omitted when cfg_scale == 1.0
• Test that a clear error message is returned when negative text conditioning is omitted when cfg_scale != 1.0
• Test that the negative text prompt gets applied when cfg_scale >1.0
• Test that a collection of cfg_scale values can be provided for per-step control.
• Test that cfg_scale_start_step and cfg_scale_end_step control the range of steps that CFG is applied

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)

[psychedelicious]

How does CFG interact with guidance? Should both of these be exposed separately in the linear UI?