Add set_shift to FlowMatchEulerDiscreteScheduler

[hlky]

What does this PR do?

from diffusers import FlowMatchEulerDiscreteScheduler

scheduler = FlowMatchEulerDiscreteScheduler()
scheduler.set_timesteps(20)
print(scheduler.sigmas, scheduler.timesteps)
"""
tensor([1.0000, 0.9474, 0.8948, 0.8423, 0.7897, 0.7371, 0.6845, 0.6319, 0.5794,
        0.5268, 0.4742, 0.4216, 0.3691, 0.3165, 0.2639, 0.2113, 0.1587, 0.1062,
        0.0536, 0.0010, 0.0000]) tensor([1000.0000,  947.4211,  894.8421,  842.2632,  789.6843,  737.1052,
         684.5263,  631.9474,  579.3684,  526.7895,  474.2105,  421.6316,
         369.0526,  316.4737,  263.8947,  211.3158,  158.7368,  106.1579,
          53.5789,    1.0000])
"""

scheduler.set_shift(7.0)
scheduler.set_timesteps(20)
print(scheduler.sigmas, scheduler.timesteps)
"""
tensor([1.0000, 0.9921, 0.9835, 0.9739, 0.9633, 0.9515, 0.9382, 0.9232, 0.9060,
        0.8863, 0.8633, 0.8361, 0.8037, 0.7642, 0.7151, 0.6522, 0.5691, 0.4540,
        0.2838, 0.0070, 0.0000]) tensor([1000.0000,  992.1342,  983.4892,  973.9432,  963.3475,  951.5190,
         938.2291,  923.1893,  906.0295,  886.2676,  863.2631,  836.1465,
         803.7072,  764.2072,  715.0599,  652.2395,  569.1180,  453.9579,
         283.8139,    6.9583])
"""

Fixes #8501

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@yiyixuxu

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[yiyixuxu]

this is actually something I would like to be able to easily handle in the refactor PR you have here https://github.com/huggingface/diffusers/pull/10146/files,i.e. we can make a special sigma schedule for hunyuan that shift dynamically

[yiyixuxu]

same as the dynamic shift in flux/sd3, it is nice we added support in both, but with the refactor, what I have in mind is that we don't have to change the pipeline, they just use flux schedule with sd3 pipeline

[yiyixuxu]

ok to add this for now if you feel strongly about this! but I think this is something we will have to refactor out later

[hlky]

I felt it was useful to have for the next version with HunyuanVideo, we can do this in the scheduler refactor but the design isn't finalised and there's no feedback yet so we probably won't get that in by the next version at the end of the week.

[yiyixuxu]

let's merge then:)

[hlky]

Just to note this is already seeing some usage #10136 (comment) pipe.scheduler._shift = 7.0 it's much more convenient for users with one-off scripts like this than importing the scheduler and creating it again from pipe.scheduler.config with shift=....