Minor API change, bug-fixes and improved test-coverage

Added

• Independent state_dict and load_state_dict functions.
• Added nonempty check for aggregation functions in Parallel.
• update_state argument to all forward_step(s) methods.
• Additional tests for edge-cases

Changed

• Changed default pad_end value to False.

Fixed

• Continual interface and conversion to support both class and module.
• Replicate padding in co._ConvNd