Created by: Xreki

Port Fully Connected operator to the new framework. The computation must be totally the same as the old FullyConnectedLayer, that is Out = sum_n{X_i * W_i} + b.

• Support different kinds of activation functions, such as identity, sigmoid, softmax.
• Implemented by calling other operators (MulOp, RowwiseAddOp, activation ops).
• Support multiple inputs and weights.

Problems I met during porting FCOp are recorded in several issues:

• #3899 (closed)
• #3922 (closed)