code: def forward(self, x):
      x = self.conv1(x)
      x = self.conv2(x)
      x = self.conv3(x)
      alpha = self.alpha_pred(x)
  
      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 Refine obj's underlying member method code with example usage: obj.forward(x) and returns: alpha