Created by: DannyIsFunny

Add 4 method to LightPredictor and CxxPredictor: 1. vector<std::string>GetInputNames() 2. vector<std::string>GetOutPutNames()
3. lite::tensor GetInputByName(const std::string name) 4. const liteTensor GetTensor(const std::string name)

we can setInput by name and getTensor by name

lite::tensor input1 = predictor.GetInputByName(predictor.GetInputNames()[0])
const lite::tensor output1 = predictor.GetTensor(predictor.GetOutputNames()[0])

(Related tests are added into paddle_api_test.cc)