code: def fit(self, train_x, train_y):
          
          if self.first_fitted:
              self.incremental_fit(train_x, train_y)
          else:
              self.first_fit(train_x, train_y)
docs: Fit the regressor with more data.
          Args:
              train_x: A list of NetworkDescriptor.
              train_y: A list of metric values.