During training the net, i got this RuntimeError: probability tensor contains either inf, nan or element < 0

[misayllk]

I have used the train_net.py, but some problems appeared,
first i got the 'line 68, in forward wt.index_copy(0, ind, torch.multinomial(prob_prev, 1).view(-1).index_select(0, ind))
RuntimeError: invalid multinomial distribution (sum of probabilities <= 0)' ,but when i cleared the memory and re-run the train_net_py，it could work successfully until the 10 epoch
In 10th epoch,i got this:
File "D:\llk\xmodaler-master\xmodaler\modeling\meta_arch\rnn_att_enc_dec.py", line 68, in forward
wt.index_copy(0, ind, torch.multinomial(prob_prev, 1).view(-1).index_select(0, ind)) #####bug
RuntimeError: probability tensor contains either inf, nan or element < 0
Can somebody help me??😥