Get the params for an optionally nested action distribution.

Only returns parameters that have tf.Tensor values.

nested_distribution The nest of distributions whose parameter tensors to extract.
legacy_distribution_network Whether this distribution was generated by a (legacy) DistributionNetwork. If False, expect the distribution was generated by a regular Network object that emits distributions.

A nest of distribution parameters. Each leaf is a dict corresponding to one distribution, with keys as parameter name and values as tensors containing parameter values.