target = None error while tuing an autotvm task

[huochaitiantang]

After PR7534(#7534), while tuning an autotvm task, self.target = None, self.target_host = None (default) in class tvm.autotvm.task.Task, will introduce the error :

@zxybazh

Traceback (most recent call last):
  File "test_quantize.py", line 66, in <module>
    tuner.tune(20, logfile=logfile)
  File "~/Desktop/tvms/tvm-oplib/tvmoplib/tvm_tuner/onnx_tuner.py", line 215, in tune
    tuner.load_history(tvm.autotvm.record.load_from_file(tmp_log_file))
  File "~/Desktop/tvms/tvm/python/tvm/autotvm/tuner/model_based_tuner.py", line 294, in load_history
    success = base_model.fit_log(data_set, self.plan_size)
  File "~/Desktop/tvms/tvm/python/tvm/autotvm/tuner/xgboost_cost_model.py", line 250, in fit_log
    res = pool.map(feature_extract_func, data)
  File "~/.pyenv/versions/3.6.4/lib/python3.6/multiprocessing/pool.py", line 266, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
  File "~/.pyenv/versions/3.6.4/lib/python3.6/multiprocessing/pool.py", line 644, in get
    raise self._value
  File "~/.pyenv/versions/3.6.4/lib/python3.6/multiprocessing/pool.py", line 424, in _handle_tasks
    put(task)
  File "~/.pyenv/versions/3.6.4/lib/python3.6/multiprocessing/connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "~/.pyenv/versions/3.6.4/lib/python3.6/multiprocessing/reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
  File "~/Desktop/tvms/tvm/python/tvm/autotvm/task/task.py", line 179, in __getstate__
    self.target, self.target_host
  File "~/Desktop/tvms/tvm/python/tvm/target/target.py", line 197, in check_and_update_host_consist
    target = Target(target, host)
  File "~/Desktop/tvms/tvm/python/tvm/target/target.py", line 98, in __init__
    raise ValueError("target has to be a string or dictionary.")
ValueError: target has to be a string or dictionary.

[zxybazh]

Hi, this is a case when the target and target host are both empty, the Target constructor function would fail. I have patched the case in the new PR. Thanks for letting me know!