Add flag to add singleton event dimension to GaussianMixture (for multivariate=False).
Add flag to add singleton event dimension to GaussianMixture (for multivariate=False).

PiperOrigin-RevId: 356255642
Change-Id: I5f4ca7f19a9840a98509c8f8faf6431fcffce5ef

deepmind/acme,deepmind/acme