fix bug in smartparam example

examples/trials/mnist-smartparam/mnist.py

@@ -223,6 +223,7 @@ def generate_defualt_params():
 
 if __name__ == '__main__':
     try:
+        nni.get_next_parameter()
         main(generate_defualt_params())
     except Exception as exception:
         logger.exception(exception)

src/sdk/pynni/nni/trial.py

@@ -47,6 +47,7 @@ def get_next_parameter():
     return _params['parameters']
 
 def get_current_parameter(tag):
+    global _params
     if _params is None:
         return None
     return _params['parameters'][tag]