Extend the AvatarMetricReporter so it can index into a tensor when creating a custom observation.
Extend the AvatarMetricReporter so it can index into a tensor when creating a custom observation.

PiperOrigin-RevId: 473214865
Change-Id: I1bf5c951177de62c2ae2d6e138fa3620f5ee7443

deepmind/meltingpot,deepmind/meltingpot