Add CheXpert dataset for vision
### Feature request

### Name 

**CheXpert: A Large Chest Radiograph Dataset with Uncertainty Labels and Expert Comparison**

### Paper

https://arxiv.org/abs/1901.07031

### Data

https://stanfordaimi.azurewebsites.net/datasets/8cbd9ed4-2eb9-4565-affc-111cf4f7ebe2

### Motivation

CheXpert is one of the fundamental models in medical image classification and can serve as a viable pre-training dataset for radiology classification or low-scale ablation / exploratory studies.

This could also serve as a good pre-training dataset for Kaggle competitions.

### Your contribution

Would love to make a PR and pre-process / get this into 🤗