fix paddle2onnx install (#1376)

paddleslim/common/load_model.py

@@ -207,7 +207,7 @@ def export_onnx(model_dir,
     if not params_filename:
         params_filename = 'model.pdiparams'
     try:
-        pkg.require('paddle2onnx')
+        os.system(' python -m pip install -U paddle2onnx==1.0.0rc4')
     except:
         from pip._internal import main
         main(['install', 'paddle2onnx==1.0.0rc4'])