Feature request: ImageNet data loader

ImageNet is quite large and locked behind [terms of access that require an account](https://www.image-net.org/download.php).

However it would be nice to be able to either
* set a config (or `ENV`) variable to download ImageNet through MLDatasets
* point MLDatasets to a local copy of ImageNet

and be able to use MLDatasets' interface of
```julia
train_x, train_y = ImageNet.traindata()
test_x, test_y = ImageNet.testdata()
```
as well as `ImageNet.convert2image(x)`.
Ideally data would be in WHCN format for Flux and Metalhead models.