Resnet
# What does this PR do?
This WIP PR adds [ResNet](https://arxiv.org/abs/1512.03385). 

Currently, the model can be used as follows

```python
import requests
from io import BytesIO
res = requests.get('https://github.com/huggingface/transformers/blob/master/tests/fixtures/tests_samples/COCO/000000039769.png?raw=true')
image = Image.open(BytesIO(res.content))

feature_extractor = AutoFeatureExtractor.from_pretrained("Francesco/resnet50")
model = ResNetForImageClassification.from_pretrained("Francesco/resnet50").eval()

inputs = feature_extractor(image, return_tensors="pt")
outputs = model(**inputs)
print(model.config.id2label[torch.argmax(outputs.logits).item()])
# tiger cat
```
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15770). All of your documentation changes will be reflected on that endpoint.<|||||>I've removed the `regression` loss from `ForImageClassification` since it is not a loss used for image classification. This open for discussion<|||||>I've removed the `D` variant code, it will be added by a different PR. Three new models outputs have been added: `BaseModelOutputWithNoAttention`, `BaseModelOutputWithNoAttentionAndWithPooling` and `ImageClassificationModelOutput`<|||||>Address most of the comments, the blocking ones are related to outputs docstring