message: Added a "note" in tf.where documentation suggesting a workaround for issue
diff: @@ -4487,6 +4487,21 @@ def where_v2(condition, x=None, y=None, name=None):
  <tf.Tensor: shape=(4,), dtype=int32, numpy=array([100, 100, 100, 100],
  dtype=int32)>
  + Note that if the gradient of either branch of the tf.where generates
  + a NaN, then the gradient of the entire tf.where will be NaN.
  + A workaround is to use an inner tf.where to ensure the function has
  + no asymptote, and to avoid computing a value whose gradient is NaN by
  + replacing dangerous inputs with safe inputs.
  +
  + Instead of this
  +
  + >>> y = -1
  + >>> tf.where(y > 0, tf.sqrt(y), y)
  +
  + Use this
  +
  + >>> tf.where(y > 0, tf.sqrt(tf.where(y > 0, y, 1)), y)
  +
  Args:
  condition: A `tf.Tensor` of type `bool`
  x: If provided, a Tensor which is of the same type as `y`, and has a shape
  