Add backward() functions for element-wise operators

At least for Add/Sub/Mul/Div.

Some of these functions will be needed if we want to have the backward of Snn Leaky Operator.

• Backward for Mul : Done in !122 (merged)
• Backward for Div : Done in !134 (merged)
• Backward for Sub : Done in !138 (merged)
• Backward for Add : Done in !137 (merged)