Avoid unnecessary DDP synchronization when gradient_accumulation_steps > 1 (#7742)

* use DDP no_sync when possible

* fix is_nlp_available addition mistake

* reformat trainer.py

* reformat trainer.py

* drop support for pytorch < 1.2

* return support for pytorch < 1.2

src/transformers/trainer.py

@@ -101,6 +101,11 @@ else:
     _use_native_amp = True
     from torch.cuda.amp import autocast
 
+if version.parse(torch.__version__) < version.parse("1.2"):
+    _use_ddp_no_sync = False
+else:
+    _use_ddp_no_sync = True
+
 if is_datasets_available():
     import datasets
 
@@ -687,6 +692,14 @@ class Trainer:
                 if (step + 1) % self.args.gradient_accumulation_steps == 0:
                     self.control = self.callback_handler.on_step_begin(self.args, self.state, self.control)
 
+                if (
+                    ((step + 1) % self.args.gradient_accumulation_steps != 0)
+                    and self.args.local_rank != -1
+                    and _use_ddp_no_sync
+                ):
+                    with model.no_sync():
+                        tr_loss += self.training_step(model, inputs)
+                else:
                     tr_loss += self.training_step(model, inputs)
                 self._total_flos += self.floating_point_ops(inputs)