message: Add usage example for tf.math.polyval
diff: @@ -4238,6 +4238,24 @@ def polyval(coeffs, x, name=None):
  A `tensor` of the shape as the expression p(x) with usual broadcasting
  rules for element-wise addition and multiplication applied.
  + Usage Example:
  +
  + >>> y = tf.math.polyval([2, 1, 0], 3) # evaluates 2 * (3**2) + 1 * (3**1) + 0 * (3**0)
  + >>> print(y)
  + tf.Tensor(21, shape=(), dtype=int32)
  +
  + `tf.math.polyval` can also be used in polynomial regression. Taking advantage of this
  + function can facilitate writing a polynomial equation as compared to explicitly writing
  + it out, especially for higher degree polynomials.
  +
  + >>> x = tf.constant(3)
  + >>> theta1 = tf.Variable(2)
  + >>> theta2 = tf.Variable(1)
  + >>> theta3 = tf.Variable(0)
  + >>> y = tf.math.polyval([theta1, theta2, theta3], x)
  + >>> print(y)
  + tf.Tensor(21, shape=(), dtype=int32)
  +
  @compatibility(numpy)
  Equivalent to numpy.polyval.
  @end_compatibility
  