Add flow shift parameter (for SD3 and Wan)

[stduhpf]

Targets #778 but could easily be applied to master for SD3.x alone.

Examples:

T2I

model	shift 1.5	shift 3	shift 6	shift 12	shift 24
Wan2.2 T2V (with 20 total steps, using #779)
Wan2.2 T2V (with 10+10 steps)
Wan2.2 TI2V 5B (40 steps)
SD3.5 medium (20 steps)
SD3.5 Large Turbo (6 steps)
SD3.5 Large (20 steps)

(note that when using 10+10 steps on Wan2.2 A14B, sampling falls out of distribution for extreme shift values, resulting in very poor quality. Using the automatic switching step introduced in #779 fixes that )

T2V

model	shift 3	shift 6	shift 12
Wan2.2 T2V (with 20 total steps, using #779)
Wan2.2 T2V (with 10+10 steps)
Wan2.2 TI2V 5B (40 steps)

[leejet]

Thank you for your contribution.