Store vae.config.scaling_factor to prevent missing attr reference (sdxl advanced dreambooth training script)

[Teriks]

In sdxl advanced dreambooth training script

vae.config.scaling_factor becomes inaccessible after: del vae

when: --cache_latents, and no --validation_prompt

What does this PR do?

Fixes a non-existent attribute reference with a particular argument combination.

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Who can review?

@sayakpaul

[sayakpaul]

Thanks!

[linoytsaban]

LGTM, thanks!

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