don't log a value for domains with no data on a step

src/levanter/doremi.py

@@ -184,7 +184,8 @@ def doremi_step(state: DoremiState, ref, batch, domains):
                 "train/mean_excess_loss": mean_excess_loss,
                 "train/mean_proxy_loss": mean_proxy_loss,
                 **{f"alpha/{domain}": weight for domain, weight in alpha_dict.items()},
-                **{f"train/{domain}/loss": loss for domain, loss in per_domain_dict.items()},
+                # just skip domains with no excess loss
+                **{f"train/{domain}/excess_loss": loss for domain, loss in per_domain_dict.items() if loss > 0},
             },
             step=state._step,
         )