Fix wrong param types, docs, and handles noise=None in scale_noise of FlowMatching schedulers

[Promisery]

What does this PR do?

In the scale_noise functions of all flow matching schedulers, the definitions and docs are wrong:

• timestep should be a torch.FloatTensor, not a float or int
• noise is not included in the document

Also, noise should be a required arg

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@yiyixuxu

[yiyixuxu]

thanks for the PR
I left one feedback

[yiyixuxu]

can we noise a required arg instead?
to properly generate noise inside scale_noise we need to pass a generator too, I don't think we need to do that

[Promisery]

Agreed

[HuggingFaceDocBuilderDev]

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