Minor bug fix: changing train_step in examples code to take a mean of…

… the stats instead of taking from the first device. Because the optimizer syncs its own stats (like loss), this didn't matter except for stats returned from the kfac_jax optimizer (or Optax optimizers using OptaxWrapper). However, the Polyak averaged loss wasn't actually synced across devices (as its not part of the optimizer anymore), so "loss_polyak" was being reported only for the first device.

PiperOrigin-RevId: 700710272

examples/training.py

@@ -633,7 +633,7 @@ def train_step(self, global_step: Array, rng: PRNGKey) -> dict[str, Numeric]:
       for i in range(gathered_stat.shape[0]):
         stats[f"{name}_{i}"] = jnp.array([gathered_stat[i]])
 
-    return kfac_jax.utils.get_first(stats)
+    return jax.tree_util.tree_map(functools.partial(jnp.mean, axis=0), stats)
 
   #                  _
   #   _____   ____ _| |
@@ -774,26 +774,35 @@ def run_evaluation(
             global_step, self._params, self._state, self._opt_state, key, batch)
 
         if params_polyak is not None:
+
           stats_no_polyak = stats
+
           stats = self.eval_batch_pmap(
               global_step, params_polyak, func_state_polyak, self._opt_state,
               key, batch)
+
           stats.update(
               {k + "_no_polyak": v for k, v in stats_no_polyak.items()
                if k != "data_seen"})
 
         if params_schedule_free is not None:
+
           stats_no_sf = stats
+
           stats = self.eval_batch_pmap(
               global_step, params_schedule_free, func_state_schedule_free,
               self._opt_state, key, batch)
+
           stats.update(
               {k + "_no_sf": v for k, v in stats_no_sf.items()
                if k != "data_seen"})
 
         averaged_stats.add(stats, 1)
 
-      # Extract all stats
+      # Extract all stats.
+      # Note that MultiChunkAccumulator.value will perform a pmean
+      # automatically, so it's fine to call "get_first" here instead of taking
+      # the mean.
       for k, v in averaged_stats.value.items():  # pytype: disable=attribute-error
         all_stats[f"{name}_{k}"] = kfac_jax.utils.get_first(v)