Fix installation on macOS.

[csukuangfj]

To fix the following error when running

python3 -c "import optimized_transducer; print(optimized_transducer.__version__)"

on macOS:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/fangjun/py38/lib/python3.8/site-packages/optimized_transducer/__init__.py", line 1, in <module>
    from .transducer_loss import TransducerLoss, transducer_loss  # noqa
  File "/Users/fangjun/py38/lib/python3.8/site-packages/optimized_transducer/transducer_loss.py", line 3, in <module>
    import _optimized_transducer
ImportError: dlopen(/Users/fangjun/py38/lib/python3.8/site-packages/_optimized_transducer.cpython-38-darwin.so, 2): Symbol not found: _THPVariableClass
  Referenced from: /Users/fangjun/py38/lib/python3.8/site-packages/_optimized_transducer.cpython-38-darwin.so
  Expected in: flat namespace
 in /Users/fangjun/py38/lib/python3.8/site-packages/_optimized_transducer.cpython-38-darwin.so