class_docstr: RationalQuadratic Kernel.
  
  This kernel function has the form:
  
  ```none
  k(x, y) = amplitude**2 * (1. + ||x - y|| ** 2 / (
    2 * scale_mixture_rate * length_scale**2)) ** -scale_mixture_rate
  ```
  
  where the double-bars represent vector length (i.e. Euclidean, or L2 Norm).
  This kernel acts over the space `S = R^(D1 x D2 .. Dd)`. When
  `scale_mixture_rate` tends towards infinity, this kernel acts like an
  ExponentiatedQuadratic kernel.
  
  The name `scale_mixture_rate` comes from the interpretation that that this
  kernel is an Inverse-Gamma weighted mixture of ExponentiatedQuadratic Kernels
  with different length scales.
  
  More formally, if `r = ||x - y|||`, then:
  
  ```none
  integral from 0 to infinity (k_EQ(r | sqrt(t)) p(t | a, a * l ** 2)) dt =
    (1 + r**2 / (2 * a * l ** 2)) ** -a = k(r)
  ```
  
  where:
    * `a = scale_mixture_rate`
    * `l = length_scale`
    * `p(t | a, b)` is the Inverse-Gamma density.
      https://en.wikipedia.org/wiki/Inverse-gamma_distribution
    * `k_EQ(r | l) = exp(-r ** 2 / (2 * l ** 2)` is the ExponentiatedQuadratic
      positive semidefinite kernel.
  
  #### References
  
  [1]: Filip Tronarp, Toni Karvonen, and Simo Saarka. Mixture
       representation of the Matern class with applications in state space
       approximations and Bayesian quadrature. In _28th IEEE International
       Workshop on Machine Learning for Signal Processing_, 2018.
       https://acris.aalto.fi/ws/portalfiles/portal/30548539/ELEC_Tronarp_etal_Mixture_Presentation_of_the_Matern_MLSP2018.pdf
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docstr: scale_mixture_rate parameter.
docstr_tok: ["scale_mixture_rate","parameter","."]
code: def scale_mixture_rate(self):
      """scale_mixture_rate parameter."""
      return self._scale_mixture_rate
  
imports: import tensorflow.compat.v2
  from tensorflow_probability.python.internal import assert_util
  from tensorflow_probability.python.internal import dtype_util
  from tensorflow_probability.python.internal import parameter_properties
  from tensorflow_probability.python.internal import tensor_util
  from tensorflow_probability.python.math.psd_kernels import positive_semidefinite_kernel
  from tensorflow_probability.python.math.psd_kernels.internal import util
label_desc: Use image node_id 9 for calling the RationalQuadratic obj's underlying member method code with example usage: obj.scale_mixture_rate() and returns: self