[Examples] Multiple enhancements to the ControlNet training scripts (#7096)
* log_validation unification for controlnet.
* additional fixes.
* remove print.
* better reuse and loading
* make final inference run conditional.
* Update examples/controlnet/README_sdxl.md
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
* resize the control image in the snippet.
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Co-authored-by: Pedro Cuenca <pedro@huggingface.co>