`accelerate` support for `OwlViT`
# What does this PR do?

This PR adds `accelerate` support for `OwlViT` so that any model from this family can be loaded in `8-bit`! Here is a small snippet on how to load and run the model in `8-bit` (based on the snippet from the model card):
```
# pip install accelerate bitsandbytes
import requests
from PIL import Image
import torch

from transformers import OwlViTProcessor, OwlViTForObjectDetection

processor = OwlViTProcessor.from_pretrained("google/owlvit-base-patch32")
model = OwlViTForObjectDetection.from_pretrained("google/owlvit-base-patch32", device_map="auto", load_in_8bit=True)

url = "http://images.cocodataset.org/val2017/000000039769.jpg"
image = Image.open(requests.get(url, stream=True).raw)
texts = [["a photo of a cat", "a photo of a dog"]]
inputs = processor(text=texts, images=image, return_tensors="pt")
outputs = model(**inputs)

# Target image sizes (height, width) to rescale box predictions [batch_size, 2]
target_sizes = torch.Tensor([image.size[::-1]])
# Convert outputs (bounding boxes and class logits) to COCO API
results = processor.post_process(outputs=outputs, target_sizes=target_sizes)

i = 0  # Retrieve predictions for the first image for the corresponding text queries
text = texts[i]
boxes, scores, labels = results[i]["boxes"], results[i]["scores"], results[i]["labels"]

# Print detected objects and rescaled box coordinates
score_threshold = 0.1
for box, score, label in zip(boxes, scores, labels):
    box = [round(i, 2) for i in box.tolist()]
    if score >= score_threshold:
        print(f"Detected {text[label]} with confidence {round(score.item(), 3)} at location {box}")
>>> Detected a photo of a cat with confidence 0.705 at location [321.25, 19.8, 643.12, 376.88]
>>> Detected a photo of a cat with confidence 0.729 at location [0.94, 53.55, 319.69, 473.91]
```
Added also a slow test to make sure users can run inference in `fp16` (`8bit` converts the model in. `fp16` under the hood)

All slow tests pass except the one mentioned in #20410 that should be fixed once merged

cc @alaradirik @sgugger @NielsRogge 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks everyone! 