Implement a bilinear initializer for transposed convolution.

[qingqing01]

This is a common use case in transposed convolution to act as upsampling.

factor = 2
w_attr = ParamAttr(learning_rate=0., regularizer=L2Decay(0.),
                  initializer=Bilinear())
conv_up = fluid.layers.conv2d_transpose(
     input,
      num_filters=C,
      output_size=None,
      filter_size=2 * factor - factor % 2,
      padding=ceil((factor - 1) / 2.),
      stride=factor,
      groups=C,
      param_attr=w_attr,
      bias_attr=False)