Image downloaders accept as = "list" to return one shared, shallow
structure. It has data, a matrix with one image per row; meta, a data
frame with matching rows; image_dim, named image dimensions; channel_order;
and source, a list containing the dataset name and acquisition URL.
Details
Metadata names are lower case: label, description, split, id,
object, and pose are used when applicable. Dataset-specific source fields
remain in meta. split records train/test identity explicitly. The legacy
data-frame result remains available with as = "data.frame" and retains its
historical column names.