Update in NeuralNetLearner function in learnign.py (#1019)

* Update in NeuralNetLearner function

* made the changes as suggested

Author: thesagarsehgal

learning.py

@@ -654,15 +654,14 @@ def predict(example):
 # ______________________________________________________________________________
 
 
-def NeuralNetLearner(dataset, hidden_layer_sizes=None,
+def NeuralNetLearner(dataset, hidden_layer_sizes=[3],
                      learning_rate=0.01, epochs=100, activation = sigmoid):
     """Layered feed-forward network.
     hidden_layer_sizes: List of number of hidden units per hidden layer
     learning_rate: Learning rate of gradient descent
     epochs: Number of passes over the dataset
     """
 
-    hidden_layer_sizes = hidden_layer_sizes or [3]  # default value
     i_units = len(dataset.inputs)
     o_units = len(dataset.values[dataset.target])