message: Update backend.py
  Adding missing import files in the commented examples. When trying out that particular example in commented section the TensorFlow and bumpy imports are missing
diff: @@ -773,6 +773,8 @@ def is_keras_tensor(x):
  Examples:
  ```python
  + >>> import tensorflow as tf
  + >>> import numpy
  >>> from keras import backend as K
  >>> from keras.layers import Input, Dense
  >>> np_var = numpy.array([1, 2])
  