[Feature Request] Optimize step sweeping

[full-diffusion-alchemist]

Is your feature request related to a problem? Please describe.
When sweeping on steps (such as when using txt2img with the X/Y plot), a new image is generated for each new step value which takes time and compute.

Describe the solution you'd like
If sweeping on steps (such as steps 20-100 (+10)), a single image with 100 steps can be computed and the intermediary images can be saved along the way as it iterates rather than creating 8 or so images from scratch. This would be the difference of only running 100 iterations vs 540 iterations (20 + 30 + 40 + 50 + 60 ...)

Describe alternatives you've considered
Haven't thought of any alternatives, open to other optimizations though.

Additional context
Add any other context or screenshots about the feature request here.

[AUTOMATIC1111]

#41