feat: denoise strength as starting noise level

[stduhpf]

Summary

Currently, in img2img mode, the strength parameter simply determines how many denoising steps to skip (e.g., a strength of 0.7 skips the first 30% of steps). This is a simple and familliar behavior, probably inherited from legacy engines like Automatic1111.

The issue with this approach is that it doesn't align with the intuitive meaning of "denoising strength," and the actual amount of noise added is heavily dependent on the specific scheduler being used.

This PR introduces an alternative interpretation of the strength parameter, treating it as a noise level rather than a step count. When enabled, a strength of 0.7 ensures that denoising begins at the step where the model's input is approximately a mix of 70% noise and 30% initial image.

Key Changes:

• Noise-based Strength: Allows strength to control the actual noise ratio. Depending on the scheduler and model, this may produce significantly different results than the default step-skipping method.
• Sigma Adjustment: Added an optional mechanism to edit the first step's sigmas to exactly match the requested noise level. Since this modifies the schedule, it is disabled by default.
• Backward Compatibility: To avoid breaking existing workflows, these features are disabled by default.

Configuration:
These can be enabled via the --extra-sample-args argument using the following boolean fields:

• strength_as_noise_level
• force_first_sigma

Related Issue / Discussion

N/A

Additional Information

With dreamshaper v8 (unet/diffusion):

Default behavior:

strength	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
default scheduler
karras scheduler

Enabling strength_as_noise_level=on:

strength	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
default scheduler
karras scheduler

Enabling strength_as_noise_level=on,force_first_sigma=on:

strength	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
default scheduler
karras scheduler

With Flux2 Klein 4b (DiT/Flow):

strength	0.1	0.2	0.3	0.4	0.5	0.6	0.7	0.8	0.9
default
noise level
+ first sigma

Checklist

• I have read and confirmed this PR follows the contribution guidelines.