implement a simplified Dense Layer

### Question 1

Please implement a simplified `Dense Layer` in `dense_layer.py`

1. Forward

$Z = A^{[i-1]} \cdot W + b$



$A^{[i-1]}: \text {previous inputs}$



2. Backward (backpropagation)

$d W= A^{[i-1] T} \cdot dZ \\ db = \sum_{j=1}^{m} d Z^{j}\\dA^{[i-1]}=dZ \cdot W^{T}$



$A^{[i-1]}: \text {previous inputs} \\ dZ: \text{previous gradients}$



Unit test is in `test_dense_layer.py`