title: Pytorch stateful lstm symbol not found
body: <p>I am getting the following error when importing StatefulUnidirectionalLSTM from pytorch_stateful_lstm :</p>
  
  <pre><code>  File "&lt;input&gt;", line 1, in &lt;module&gt;
    File "/Applications/PyCharm.app/Contents/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
      module = self._system_import(name, *args, **kwargs)
    File "/Users/user/PycharmProjects/venv/lib/python3.7/site-packages/pytorch_stateful_lstm/__init__.py", line 11, in &lt;module&gt;
      from _pytorch_stateful_lstm import (
    File "/Applications/PyCharm.app/Contents/helpers/pydev/_pydev_bundle/pydev_import_hook.py", line 21, in do_import
      module = self._system_import(name, *args, **kwargs)
  ImportError: dlopen(/Users/user/PycharmProjects/venv/lib/python3.7/site-packages/_pytorch_stateful_lstm.cpython-37m-darwin.so, 2): Symbol not found: __ZN3c1019UndefinedTensorImpl10_singletonE
    Referenced from: /Users/user/PycharmProjects/venv/lib/python3.7/site-packages/_pytorch_stateful_lstm.cpython-37m-darwin.so
    Expected in: flat namespace
   in /Users/user/PycharmProjects/venv/lib/python3.7/site-packages/_pytorch_stateful_lstm.cpython-37m-darwin.so
  </code></pre>
  
  <p>My environment information is:</p>
  
  <pre><code>Is debug build: No
  CUDA used to build PyTorch: None
  
  OS: Mac OSX 10.14.6
  GCC version: Could not collect
  CMake version: Could not collect
  
  Python version: 3.7
  Is CUDA available: No
  CUDA runtime version: No CUDA
  GPU models and configuration: No CUDA
  Nvidia driver version: No CUDA
  cuDNN version: No CUDA
  
  Versions of relevant libraries:
  [pip3] numpy==1.18.2
  [pip3] pytorch-fast-elmo==0.6.12
  [pip3] pytorch-stateful-lstm==1.6.0
  [pip3] torch==1.0.1.post2
  [conda] Could not collect
  </code></pre>
  
  <p>Has any one encountered this error and how to fix it? </p>