add image-classification task template
Awesome! Thanks for adding a new task template - great work @nateraw  🚀 !
Snippet below is the tl;dr, but you can try it out directly here:

[![Open In Collab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/gist/nateraw/005c025d41f0e48ae3d4ee61c0f20b70/image-classification-task-template-demo.ipynb)

```python
from datasets import load_dataset

ds = load_dataset('nateraw/image-folder', data_files='PetImages/')
# DatasetDict({
#     train: Dataset({
#         features: ['file', 'labels'],
#         num_rows: 23410
#     })
# })

ds = ds.prepare_for_task('image-classification')
# DatasetDict({
#     train: Dataset({
#         features: ['image_file_path', 'labels'],
#         num_rows: 23410
#     })
# })
```