distributions.mvn_diag using deprecated scale_identity_multiplier kwarg

[stewartmiles]

TFP 0.19.0 is using a deprecated TFP kwarg scale_identity_multiplier which results in the following log spam:

From site-packages/tensorflow_probability/python/distributions/distribution.py:342: calling MultivariateNormalDiag.__init__ (from tensorflow_probability.python.distributions.mvn_diag) with scale_identity_multiplier is deprecated and will be removed after 2020-01-01.
Instructions for updating:
`scale_identity_multiplier` is deprecated; please combine it into `scale_diag` directly instead.

Related tensorflow/tensorflow#55190

[ColCarroll]

thanks! i'm taking a look at this.