segmentation fault issue

```
(tensorflow09GPU)➜ MemN2N-tensorflow git:(master) python main.py --nhop 6 --mem_size 100
I tensorflow/stream_executor/dso_loader.cc:108] successfully opened CUDA library libcublas.so locally
I tensorflow/stream_executor/dso_loader.cc:108] successfully opened CUDA library libcudnn.so locally
I tensorflow/stream_executor/dso_loader.cc:108] successfully opened CUDA library libcufft.so locally
I tensorflow/stream_executor/dso_loader.cc:108] successfully opened CUDA library libcuda.so locally
I tensorflow/stream_executor/dso_loader.cc:108] successfully opened CUDA library libcurand.so locally
Read 929589 words from data/ptb.train.txt
Read 73760 words from data/ptb.valid.txt
Read 82430 words from data/ptb.test.txt
{'batch_size': 128,
'checkpoint_dir': 'checkpoints',
'data_dir': 'data',
'data_name': 'ptb',
'edim': 150,
'init_hid': 0.1,
'init_lr': 0.01,
'init_std': 0.05,
'is_test': False,
'lindim': 75,
'max_grad_norm': 50,
'mem_size': 100,
'nepoch': 100,
'nhop': 6,
'nwords': 10000,
'show': False}
[1] 9730 segmentation fault (core dumped) python main.py --nhop 6 --mem_size 100
```

Start by running the reported `python main.py --nhop 6 --mem_size 100` command and inspect `main.py` around the point after the dataset and configuration output. Compare behavior with the CUDA libraries shown in the log; done means the command no longer exits with a segmentation fault.

Written by the indexing model from the issue text.