enable flux pipeline compatible with unipc and dpm-solver

[gameofdimension]

with this pr, we can use dpm/unipc together with flux pipeline by

pipe.scheduler = DPMSolverMultistepScheduler.from_config(
    pipe.scheduler.config, use_flow_sigmas=True, prediction_type="flow_prediction")

or

pipe.scheduler = UniPCMultistepScheduler.from_config(
    pipe.scheduler.config, use_flow_sigmas=True, prediction_type="flow_prediction")

What does this PR do?

Fixes #11907

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[yiyixuxu]

ohh thanks so much @gameofdimension
does it make sense to add use_dynamic_shifting config to dpm/unipc when use_flow_sigmas is enabled?

essentially, are we able to:

1. not adding any changes to the pipeline
2. add use_dynamic_shifting config the dpm/unipc scheduler, and accept the optional mu in their set_timesteps
3. use this API at runtime to switch pipe.scheduler = DPMSolverMultistepScheduler.from_config(pipe.scheduler.config,use_flow_sigma=True, prediction_type = "flow_prediction" )

[gameofdimension]

Noted, I'll improve that

[gameofdimension]

To resolve issue #11907, we need to set sigmas=None.