R2D2: Use Acme unroll instead of Sonnet's when an Acme RNNCore is passed.
R2D2: Use Acme unroll instead of Sonnet's when an Acme RNNCore is passed.

PiperOrigin-RevId: 322100056
Change-Id: Id2750037ffe5dadc801df12247eab5babd76220b

deepmind/acme,deepmind/acme