Set default initializer for variational distribution's stddev to be close to 0.
Set default initializer for variational distribution's stddev to be close to 0.

The defaults in passing around arguments were incorrect. In particular, originally a `None` initializer is passed to `untransformed_scale` in all mean-field variational distributions; Keras defaults this to Glorot uniform. This is bad for initializing scale parameters: converging to a non-trivial solution typically requires an initialization <-3. This is because its transform implies a stddev initialization close to 0, which allows the parameter update trajectory to begin roughly like non-VI-based gradient descent.

PiperOrigin-RevId: 192850120

tensorflow/probability,tensorflow/probability