Add imagefolder dataset
A generic imagefolder dataset inspired by `torchvision.datasets.ImageFolder`. 

Resolves #2508 

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Example Usage:

[![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/gist/nateraw/954fa8cba4ff806f6147a782fa9efd1a/imagefolder-official-example.ipynb)