code: def eval(self, alpha=1.0):
      reward = 0
      # Add here a function for shaping a reward
  
      return self.logp / float(self.leng - 1 + 1e-6) + alpha * reward
  
imports: from copy import deepcopy
  import torch
  import torch.nn
  from graph4nlp.pytorch.modules.prediction.generation.base import DecoderBase
label_desc: Use image node_id 2 for calling the BeamSearchNode obj's underlying member method code with example usage: obj.eval(alpha) and returns: unknown