Moves the utils.host_to_global_device_array call from SpmdTrainer._run_step to SpmdTrainer.run.

This makes it easier for subclasses of SpmdTrainer to override `_run_step`.

Author: ruomingp

axlearn/common/trainer.py

@@ -417,7 +417,7 @@ def _should_start_trace():
                         stop_trace_step = self.step + 3
                     self._step = self._step + 1
                     self.vlog(3, "Start step %s", self.step)
-                    output = self._run_step(input_batch)
+                    output = self._run_step(utils.host_to_global_device_array(input_batch))
                     self.vlog(3, "Done step %s", self.step)
                     num_steps += 1
                     if num_steps % 100 == 0:
@@ -688,13 +688,11 @@ def _run_step(self, input_batch: NestedTensor) -> NestedTensor:
         """Runs a single training step.
 
         Args:
-            input_batch: a NestedTensor.
+            input_batch: a NestedTensor containing global arrays.
 
         Returns:
             A dict containing 'loss' and 'aux' outputs.
         """
-        input_batch = utils.host_to_global_device_array(input_batch)
-
         with jax.profiler.StepTraceAnnotation("train", step_num=self.step):
             # Note(Jan 2022):
             # pjit currently requires all parameters to be specified as positional args.