)

Summary:
Pull Request resolved: facebookresearch#494

The `ImageNetDataset` was just a subset of `ImagePathDataset`, got rid of it. Improved the API of `ImagePathDataset` and made it a registered component which can be built. Improved the associated test cases as well.

Copied from the docstring -

```
        Only one of image_folder or image_files should be passed to specify the images.

            image_folder: A directory with one of the following structures -
                - A directory containing sub-directories with images for each target,
                    which is the format expected by
                    :class:`torchvision.datasets.ImageFolder` -

                    dog/xxx.png
                    dog/xxy.png
                    cat/123.png
                    cat/nsdf3.png

                    In this case, the targets are inferred from the sub-directories.
                - A directory containing images -

                    123.png
                    xyz.png

                    In this case, the targets are not returned (useful for inference).
            image_files: A list of image files -

                [
                    "123.png",
                    "dog/xyz.png",
                    "/home/cat/aaa.png"
                ]

                In this case, the targets are not returned (useful for inference).
```

Reviewed By: vreis

Differential Revision: D21220201

fbshipit-source-id: a0b73b2bb7a88333facfe4a24faab2cfae629e73