[Core] refactor embeddings.py so that it becomes a nice module.

[sayakpaul]

What does this PR do?

The embeddings.py script is becoming long and bloated.

This PR breaks the script meaningfully so that it can be supplemented as a module.

1. Creates a embeddings_utils.py to have common methods shared across the board.
2. Moves the IP Adapter embeddings blocks to a dedicated embeddings/ip_adapter.pt.
3. Does the same for PixArt-related embeddings, too.

This should be very much backwards-preserving.

Once I have an initial approval, I will make sure all the embedding classes are documented, too.

[HuggingFaceDocBuilderDev]

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[sayakpaul]

@yiyixuxu and @DN6 did you have a chance to think through this? :)

[yiyixuxu]

I'm cool to divide them up a little bit by "type":

• positional embeddings is a pretty big category on its own,
• timesteps
• text/image: we only have one text projection embedding so ideally just combine them; they are all simply projection layers
• other: gligen bounding box, label embedding
• combined

[sayakpaul]

Yeah that sounds reasonable to me. I will let @DN6 also comment here and I will take a stab :)

[DN6]

Agree to breaking up by type as well. @yiyixuxu's suggestions make sense to me.

[sayakpaul]

Closing because of brutal merge conflicts. Let's jam on https://github.com/huggingface/diffusers/pull/8722, instead.