[numpy] one_hot: fixed edgecase

numpy.py

@@ -3,8 +3,10 @@
 import numpy as np
 
 
-def one_hot(a):
-    o_h = np.zeros((a.size, a.max() + 1))
+def one_hot(a, n=None):
+    if n is None:
+        n = a.max() + 1
+    o_h = np.zeros((a.size, n))
     o_h[np.arange(a.size), a] = 1
     return o_h