fix nce sample code for python3, test=develop (#25103)

fix nce sample code for python3 compatability.

python/paddle/fluid/layers/loss.py

@@ -725,7 +725,7 @@ def nce(input,
 
             window_size = 5
             words = []
-            for i in xrange(window_size):
+            for i in range(window_size):
                 words.append(fluid.data(
                     name='word_{0}'.format(i), shape=[-1, 1], dtype='int64'))
 
@@ -733,7 +733,7 @@ def nce(input,
             label_word = int(window_size / 2) + 1
 
             embs = []
-            for i in xrange(window_size):
+            for i in range(window_size):
                 if i == label_word:
                     continue