[Examples] Multiple enhancements to the ControlNet training scripts

[sayakpaul]

What does this PR do?

This PR adds another call to the log_validation() function after the serialization of the trained ControlNet checkpoint. This helps to validate if the training was effective and catch issues like #7080.

[HuggingFaceDocBuilderDev]

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[pcuenca]

Nice!

[yiyixuxu]

I like the added section in the readme -
I am not sure if the rest of the change is needed here since there isn't any actual bug in #7080; they just did not resize the control_image, and I think it complicates codes quite a bit

But I'm ok with it if you think it is better this way

[sayakpaul]

I think it complicates codes quite a bit

Why do you think the code becomes more complicated? The change is also in line with what we do in other training scripts, i.e., run a final validation inference with the saved checkpoint.

[yiyixuxu]

Why do you think the code becomes more complicated? The change is also in line with what we do in other training scripts, i.e., run a final validation inference with the saved checkpoint.

ok!