[feat] Add ImageNet dataset
_The documentation is not available anymore as the PR was closed or merged._ Thanks for the comments. I believe I have addressed all of them and also decreased the size of the dummy data file, so it should be ready for a re-review. I also made a change to allow adding synset mapping and valprep script in config in case we add ImageNet 21k some time later.  @lhoestq I have updated the PR to address all of the review comments.
To use the dataset download the tar file
[imagenet_object_localization_patched2019.tar.gz](https://www.kaggle.com/competitions/imagenet-object-localization-challenge/data?select=imagenet_object_localization_patched2019.tar.gz) from Kaggle and then point the datasets library to it by using:

```py
from datasets import load_dataset

dataset = load_dataset("imagenet",
data_dir="/path/to/imagenet_object_localization_patched2019.tar.gz")
```

Currently train and validation splits are supported.