Add community class StableDiffusionXL_T5Pipeline

[ppbrown]

What does this PR do?

This adds a new community pipeline, named
StableDiffusionXL_T5Pipeline

It grafts the T5 xxl encoder onto SDXL, completely replacing TE1 and TE2.

It has been tested to produce output. However, the output isnt particularly valid.
The unet needs to be retrained.

Will initially be used with base model opendiffusionai/stablediffusionxl_t5
until a proper (re)fine-tune can be made.

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

• Pipelines and pipeline callbacks: @yiyixuxu and @asomoza
• Training examples: @sayakpaul
• Docs: @stevhliu and @sayakpaul
• JAX and MPS: @pcuenca
• Audio: @sanchit-gandhi
• General functionalities: @sayakpaul @yiyixuxu @DN6

[ppbrown]

PS: This code was tested locally and ran, using the following test harness:

from diffusers import DiffusionPipeline
import torch.nn as nn, torch, types

SDXL_DIR = "/home/phil/git/models/t5-sdxl-model"

LOCAL_CODE = "/home/phil/git/diffusers.t5/examples/community/"

pipe = DiffusionPipeline.from_pretrained(
    SDXL_DIR, custom_pipeline=LOCAL_CODE, use_safetensors=True,
    torch_dtype=torch.bfloat16,
)

print("model initialized. Now moving to CUDA")
pipe.to("cuda")

print("Trying render now...")

images = pipe("a misty Tokyo alley at night",num_inference_steps=30).images
images[0].save("test.png")

[HuggingFaceDocBuilderDev]

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[asomoza]

@bot /style

[asomoza]

thanks @ppbrown , can you add some simple information to the README so people will know what this pipeline is, just a description and maybe a link to your experiment?

[asomoza]

also the style bot didn't work, can you run make style and make quality so the test passes