Loosen class requirements in from_preset

[mattdangerw]

• Push all the preset loading logic to the preset loader class.
• Loosen restrictions on what tokenizers/image converts can be loaded with what backbones.
• Fix some bugs with converters.
• Pass user kwargs around explicitly as dicts instead of using **kwargs everywhere (which is less safe).

[mattdangerw]

This will probably have some failing tests, just seeing how it does. Need to add some unit tests for a few bug I spotted in our converters still.