Add solver and loss

[aidangomez]

• Added L2 Loss Layer and LossLayer protocol
• Added SGDSolver and Solver protocol

Solver contains a Runner contains a ForwardRunnerInstance and BackwardRunnerInstance.

• ForwardRunnerInstance is responsible for input/output data
• BackwardRunnerInstance is responsible for input/output diff
• Solver is responsible for updating parameters

[aidangomez]

@aleph7 Just remembered, these still need tests. So maybe hold off on review.