Fix convert_pkl_to_pb import error

[chenhangcal]

When running:
python tools/convert_pkl_to_pb.py -h

Raise an error:
Traceback (most recent call last): File "tools/convert_pkl_to_pb.py", line 44, in <module> from caffe2.caffe2.fb.predictor import predictor_exporter, predictor_py_utils ModuleNotFoundError: No module named 'caffe2.caffe2'

Change import module to fix this.
#969

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