fix issue that training flux controlnet was unstable and validation r…

[PromeAIpro]

fix issue that training flux controlnet was unstable and validation results were unstable

What does this PR do?

Fixes # (issue)

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@sayakpaul @yiyixuxu

a simple fix of examples/controlnet/train_controlnet_flux.py and update readme examples/controlnet/README_flux.md
related issue #11181 (comment)

[sayakpaul]

Thanks for the quick changes.

[HuggingFaceDocBuilderDev]

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