RegNet
# What does this PR do?
This WIP PR adds [RegNet](https://arxiv.org/abs/2003.13678). 

Currently, the model can be used as follows

```python
from transformers import  RegNetConfig, RegNetForImageClassification
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("zuppif/regnet-y-040")
model = RegNetForImageClassification.from_pretrained("zuppif/regnet-y-040").eval()

inputs = feature_extractor(image, return_tensors="pt")
outputs = model(**inputs)
print(model.config.id2label[torch.argmax(outputs.logits).item()])
# tiger cat
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>Updated the codebase to use `config` when is possible. I had to update `ResNetModel` as well to make `make fixup` happy :)<|||||>Conversion script for the 10B regnet model added + needed changes inside `PretrainedModel._load_pretrained_model_low_mem` <|||||>Thanks to all reviewers, I've rebased and updated the code accordingly 