def get_completion_probability(self, input, completion, tok):
      self.model.eval()
      ids = tok.encode(input).ids
      ids = torch.tensor(ids, device=self.device).unsqueeze(0)
      completion_ids = torch.tensor(tok.encode(completion).ids, device=self.device).unsqueeze(0)
      probs = []
      for i in range(completion_ids.size(1)):
          y = self.model(ids)
          y = y[0,:,-1].softmax(dim=0)
          # prob is the probability of this completion.
          prob = y[completion_ids[0,i]]
          probs.append(prob)
          ids = torch.cat([ids, completion_ids[:,i:i+1]], dim=1)
      #
      return torch.tensor(probs)
  #