[core] Refactor condition embedders (and others) modules from pipelines

[sayakpaul]

What does this PR do?

We have a bunch of model-level components under src/diffusers/pipelines module. This PR factors them out of the pipelines module and places them under src/diffusers/models module strategically.

For reviewers

Main changes can be summarized as follows:

• Move an existing model-level module (e.g., audio_tokenizer_ace_step.py) to src/diffusers/models and deprecate gracefully.
• I have used my best judgment to determine which model gets to either of autoencoders and condition_embedders. When I wasn't sure, I simply placed them under others. No strong opinions here.
• scripts/smoke_test_relocated_pretrained.py is for transient testing until the PR is merged. Will delete it after.
• There is a tests/models/test_relocated_class_loading.py which will check if deprecation warnings are raised and will also help to remove those deprecation paths once the cycle is over.

[HuggingFaceDocBuilderDev]

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