Deprecate ElementType

[guybuk]

Due to legacy design, the ElementType is an enum with values that sort of, but not exactly, overlap with DataCategory. In practice, ElementTypes are only semantic:

• In DualDatasets, we expect to have two image elements, but they need to have separate types to be distinguishable (e.g. ElementType == low_res or high_res in super-resolution). When we convert to PyTorch, if we don't do this, we'll have collisions (both low and high-res will be converted to the 'image' key)
• In case of DisplayEngines, two element types with the same category can be displayed differently depending on context (e.g. source and target)

The go-to solution is to just use strings, but we're not fans of magic strings everywhere. Also, there's a need to synchronize between element types in datasets (e.g. 'image', 'class_label') and display engines, so we don't get 'image' vs 'img' or 'class_label' vs 'class' mismatches.

[guybuk]

Closed with #12