Need an op to see if a tensor holds data

[ktlichkid]

We may need an op to tell if a tensor holds data or not. In our current seq2seq models, if the output sentence length < max_length, decoder will output empty tensors. Those tensors were not handled correctly in current version -- the program would crash because the while loop would continue and try looking for next words from empty tensors. As a consequence we need an op to help break the while loop if that was the case.