Add a simple D4PG example that learns OpenAI Gym's MountainCarContinuous-v0.
Add a simple D4PG example that learns OpenAI Gym's MountainCarContinuous-v0.

PiperOrigin-RevId: 316100722
Change-Id: Ia5d7d846801a30959709d9d64b0fce6fddd67ed7

deepmind/acme,deepmind/acme