AttributeError: 'str' object has no attribute 'size'

[imnunlp]

Traceback (most recent call last):
File "train.py", line 341, in
train(args, device_id)
File "train.py", line 273, in train
trainer.train(train_iter_fct, args.train_steps)
File "/users/qqrlg/.conda/envs/Bertsum/BertSum-master/src/models/trainer.py", line 157, in train
report_stats)
File "/users/qqrlg/.conda/envs/Bertsum/BertSum-master/src/models/trainer.py", line 321, in _gradient_accumulation
sent_scores, mask = self.model(src, segs, clss, mask, mask_cls)
File "/users/qqrlg/.local/lib/python3.7/site-packages/torch/nn/modules/module.py", line 1130, in _call_impl
return forward_call(*input, **kwargs)
File "/users/qqrlg/.conda/envs/Bertsum/BertSum-master/src/models/model_builder.py", line 102, in forward
sents_vec = top_vec[torch.arange(top_vec.size(0)).unsqueeze(1), clss]
AttributeError: 'str' object has no attribute 'size'