[Refactor] TensorRTStableDiffusionPipeline - Remove StableDiffusionReferencePipeline inheriting from DiffusionPipeline

[krishnakalyan3]

This pull request refactors the StableDiffusionReferencePipeline class to inherit from the DiffusionPipeline class rather than the StableDiffusionPipeline class.

Fixes # (issue)
#6984

[krishnakalyan3]

cc: @yiyixuxu

[krishnakalyan3]

Example output for this code

import torch
from diffusers import DDIMScheduler
from diffusers.pipelines import DiffusionPipeline

# Use the DDIMScheduler scheduler here instead
scheduler = DDIMScheduler.from_pretrained("stabilityai/stable-diffusion-2-1", subfolder="scheduler")

pipe = DiffusionPipeline.from_pretrained("stabilityai/stable-diffusion-2-1",
    custom_pipeline="stable_diffusion_tensorrt_txt2img",
    variant='fp16',
    torch_dtype=torch.float16,
    scheduler=scheduler,)

# re-use cached folder to save ONNX models and TensorRT Engines
pipe.set_cached_folder("stabilityai/stable-diffusion-2-1", variant='fp16',)

pipe = pipe.to("cuda")

prompt = "a beautiful photograph of Mt. Fuji during cherry blossom"
image = pipe(prompt).images[0]
image.save('tensorrt_mt_fuji.png')