message: Improve math_ops.py docs
  Added usage examples to the tf.math.cumsum() docstring. What exists currently is syntax-like declarations and not  `self contained` code samples.
diff: @@ -3290,21 +3290,36 @@ def cumsum(x, axis=0, exclusive=False, reverse=False, name=None):
  element of the input is identical to the first element of the output:
  ```python
  - tf.cumsum([a, b, c]) # [a, a + b, a + b + c]
  + # tf.cumsum([a, b, c]) # [a, a + b, a + b + c]
  + x = tf.constant([2, 4, 6, 8])
  + tf.cumsum(x) # [ 2, 6, 12, 20]
  +
  + # using varying `axis` values
  + y = tf.constant([[2, 4, 6, 8], [1,3,5,7]])
  +
  + tf.cumsum(y, axis=0) # [[ 2, 4, 6, 8],
  + # [ 3, 7, 11, 15]]
  +
  + tf.cumsum(y, axis=1) # [[ 2, 6, 12, 20],
  + # [ 1, 4, 9, 16]]
  ```
  By setting the `exclusive` kwarg to `True`, an exclusive cumsum is performed
  instead:
  ```python
  - tf.cumsum([a, b, c], exclusive=True) # [0, a, a + b]
  + # tf.cumsum([a, b, c], exclusive=True) => [0, a, a + b]
  + x = tf.constant([2, 4, 6, 8])
  + tf.cumsum(x, exclusive=True) # [ 0, 2, 6, 12]
  ```
  By setting the `reverse` kwarg to `True`, the cumsum is performed in the
  opposite direction:
  ```python
  - tf.cumsum([a, b, c], reverse=True) # [a + b + c, b + c, c]
  + # tf.cumsum([a, b, c], reverse=True) # [a + b + c, b + c, c]
  + x = tf.constant([2, 4, 6, 8])
  + tf.cumsum(x, reverse=True) # [20, 18, 14, 8]
  ```
  This is more efficient than using separate `tf.reverse` ops.
  @@ -3312,7 +3327,9 @@ def cumsum(x, axis=0, exclusive=False, reverse=False, name=None):
  The `reverse` and `exclusive` kwargs can also be combined:
  ```python
  - tf.cumsum([a, b, c], exclusive=True, reverse=True) # [b + c, c, 0]
  + # tf.cumsum([a, b, c], exclusive=True, reverse=True) # [b + c, c, 0]
  + x = tf.constant([2, 4, 6, 8])
  + tf.cumsum(x, exclusive=True, reverse=True) # [18, 14, 8, 0]
  ```
  Args:
  