code: def forward(self, fea_list, shape_list):
      x = fea_list[-1]
      x = self.deconv6(x)
      x = self.deconv5(x, fea_list[4], shape_list[4])
      x = self.deconv4(x, fea_list[3], shape_list[3])
      x = self.deconv3(x, fea_list[2], shape_list[2])
      x = self.deconv2(x, fea_list[1], shape_list[1])
      x = self.deconv1(x, fea_list[0], shape_list[0])
      alpha = self.alpha_conv(x)
      alpha = F.sigmoid(alpha)
  
      return alpha
  
imports: from collections import defaultdict
  import paddle
  import paddle.nn
  import paddle.nn.functional
  from paddleseg.models import layers
  from paddleseg import utils
  from paddleseg.cvlibs import manager
  from ppmatting.models.losses import MRSD
label_desc: Use image node_id 2 for calling the Decoder obj's underlying member method code with example usage: obj.forward(fea_list, shape_list) and returns: alpha