message: Update backend.py
  adding missing import numpy
diff: @@ -653,6 +653,7 @@ def variable(value, dtype=None, name=None, constraint=None):
  Examples:
  ```python
  + >>> import numpy as np
  >>> from keras import backend as K
  >>> val = np.array([[1, 2], [3, 4]])
  >>> kvar = K.variable(value=val, dtype='float64', name='example_var')
  