code: def forward(self, x, mid_fea):
      fea1, fea2, fea3, fea4, fea5 = mid_fea["shortcut"]
      im = mid_fea["image_fea"]
      x = self.layer1(x) + fea5  # N x 256 x 32 x 32
      x = self.layer2(x) + fea4  # N x 128 x 64 x 64
      x = self.gca(im, x, mid_fea["unknown"])  # contextual attention
      x = self.layer3(x) + fea3  # N x 64 x 128 x 128
      x = self.layer4(x) + fea2  # N x 32 x 256 x 256
      x = self.conv1(x)
      x = self.bn1(x)
      x = self.leaky_relu(x) + fea1
      x = self.conv2(x)
  
      alpha = (self.tanh(x) + 1.0) / 2.0
  
      return alpha
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from paddleseg.models import layers
  from paddleseg import utils
  from paddleseg.cvlibs import manager, param_init
  from ppmatting.models.layers import GuidedCxtAtten
label_desc: Use image node_id 2 for calling the ResGuidedCxtAtten_Dec obj's underlying member method code with example usage: obj.forward(x, mid_fea) and returns: alpha