Fix crash when running on CPU

[stevenfontanella]

This commit fixes the following crash when running python run.py --input_files input.txt --segment on CPU:

THCudaCheck FAIL file=/pytorch/aten/src/THC/THCGeneral.cpp line=51 error=30 : unknown error
Traceback (most recent call last):
  File "run.py", line 48, in <module>
    segment(args)
  File "/home/steven/code/AIA/n/src/api.py", line 134, in segment
    model = AttnSegModel(args, word_vocab)
  File "/home/steven/code/AIA/n/src/elmo_crf_seg.py", line 18, in __init__
    self.elmo = ElmoEmbedder(cuda_device=0 if args.gpu is not None else -1)
  File "/usr/local/lib/python3.6/dist-packages/allennlp/commands/elmo.py", line 166, in __init__
    self.elmo_bilm = self.elmo_bilm.cuda(device=cuda_device)
  File "/home/steven/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 260, in cuda
    return self._apply(lambda t: t.cuda(device))
  File "/home/steven/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 187, in _apply
    module._apply(fn)
  File "/home/steven/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 187, in _apply
    module._apply(fn)
  File "/home/steven/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 193, in _apply
    param.data = fn(param.data)
  File "/home/steven/.local/lib/python3.6/site-packages/torch/nn/modules/module.py", line 260, in <lambda>
    return self._apply(lambda t: t.cuda(device))
  File "/home/steven/.local/lib/python3.6/site-packages/torch/cuda/__init__.py", line 162, in _lazy_init
    torch._C._cuda_init()
RuntimeError: cuda runtime error (30) : unknown error at /pytorch/aten/src/THC/THCGeneral.cpp:51