* Use atexit to remove TempDirectory before interpreter shutdown.
 * Can't rely on complex functions from __del__ anyway.
 * Fixes warning message on my box:
       Exception ignored in: <function TempDirectory.__del__ at 0x12be10680>
       Traceback (most recent call last):
        File ".../tvm/python/tvm/contrib/util.py", line 55, in __del__
        File ".../tvm/python/tvm/contrib/util.py", line 51, in remove
        File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/shutil.py", line 509, in rmtree
        AttributeError: 'NoneType' object has no attribute 'path'