Fix the issue(keras-team#2195): Improve readability and comprehensibility of Stable Diffusion source

[y-vectorfield]

What does this PR do?

I added the casting method to Stable Diffusion source as y-vectorfield-fj(my previous account name).
However, this method is implemented abruptly and unnaturally.
Hence, I modified the source and added comments to improve readability and comprehensibility.

Who can review?

@ianstenbit
@sampathweb

[sampathweb]

@y-vectorfield - This PR only adds a comment - # Keras backend array need to cast explicitly. It may not be required. If it was adding any doc strings for the user, it might be useful. So for now, we could close this PR?

[y-vectorfield]

@y-vectorfield - This PR only adds a comment - # Keras backend array need to cast explicitly. It may not be required. If it was adding any doc strings for the user, it might be useful. So for now, we could close this PR?

@sampathweb, thank you very much for your comments and review.
I think this modification is necessary for the improvement of Keras's CV.
If we add the state-of-the-art DL models, we will refer to the previous models of this.
Hence, I thought it was necessary to improve readability and comprehensibility.
If there is no problem, please merge my modification.

[divyashreepathihalli]

Merged, Thank you!

[y-vectorfield]

@divyashreepathihalli, thank you very much!