Add CUBirds200, new vision model interface, fix clone_module for shared parameters

Added

• l2l.vision.datasets.CUBirds200.

Changed

• Optimization transforms can be accessed directly through l2l.optim, e.g. l2l.optim.KroneckerTransform.
• All vision models adhere to the .features and .classifier interface.

Fixed

• Fix clone_module for Modules whose submodules share parameters.