Hope the following is helpful:
I am using a small cluster ( 40 thread ). This problem has no related with IPython notebook because I tried running the code without IPython notebook.  And in general the code runs for several batches ( less than 10 ) and stops without any warning.
And this problem disappear after I restart the cluster.
I guess the problem may come from, either I run too many times of the nn.DataParallel, or I run too many times of pathos.multiprocessing. BTW, I am using Python 2.
Last, this problem has no related with model, at least I tried with different models.