Fix MSE calculation bug and refactor decision tree logic

Author: kazisufiyan-k2

machine_learning/decision_tree.py

@@ -96,7 +96,7 @@ def train(self, x, y):
             return
 
         best_split = 0
-        min_error = self.mean_squared_error(x, np.mean(y)) * 2
+        min_error = self.mean_squared_error(y, np.mean(y)) * 2
 
         """
         loop over all possible splits for the decision tree. find the best split.