Fix progress bar updates for Pod Training (#8258)

* Fix progress bar updates for Pod Training

* Fix progress bar updates for Pod Training

* Add _pod_progress_bar_force_stdout

pytorch_lightning/plugins/training_type/tpu_spawn.py

@@ -34,6 +34,7 @@
 from pytorch_lightning.utilities.distributed import rank_zero_only, ReduceOp, tpu_distributed
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from pytorch_lightning.utilities.seed import reset_seed
+from pytorch_lightning.utilities.types import STEP_OUTPUT
 
 if _TPU_AVAILABLE:
     import torch_xla.core.xla_env_vars as xenv
@@ -282,6 +283,26 @@ def test_step(self, *args, **kwargs):
     def predict_step(self, *args, **kwargs):
         return self.model(*args, **kwargs)
 
+    def training_step_end(self, output: STEP_OUTPUT) -> STEP_OUTPUT:
+        self._pod_progress_bar_force_stdout()
+        return output
+
+    def validation_step_end(self, output: STEP_OUTPUT) -> STEP_OUTPUT:
+        self._pod_progress_bar_force_stdout()
+        return output
+
+    def test_step_end(self, output: STEP_OUTPUT) -> STEP_OUTPUT:
+        self._pod_progress_bar_force_stdout()
+        return output
+
+    def _pod_progress_bar_force_stdout(self) -> None:
+        # Why is it required? The way `pytorch_xla.distributed` streams logs
+        # from different vms to the master worker doesn't work well with tqdm
+        # Ref: https://github.com/pytorch/xla/blob/master/torch_xla/distributed/xla_dist.py#L140
+        # The print statement seems to force tqdm to flush stdout.
+        if self.tpu_global_core_rank == 0 and int(os.getenv(xenv.TPUVM_MODE, 0)) == 1:
+            print()
+
     def save_checkpoint(self, checkpoint: Dict[str, Any], filepath: str) -> None:
         """Save model/training states as a checkpoint file through state-dump and file-write.