Add basic support for FP16 in SageMaker model parallelism (#11407)

* Add FP16 support for SageMaker MP

* Add print debugs

* Squeeze

* Remove debug statements

* Add defensive check

* Typo

src/transformers/trainer.py

@@ -412,7 +412,12 @@ class Trainer:
         if args.fp16 and not args.deepspeed:  # deepspeed manages its own fp16
             if self.fp16_backend == "amp":
                 self.use_amp = True
-                self.scaler = ShardedGradScaler() if self.sharded_ddp is not None else torch.cuda.amp.GradScaler()
+                if is_sagemaker_mp_enabled():
+                    self.scaler = smp.amp.GradScaler()
+                elif self.sharded_ddp is not None:
+                    self.scaler = ShardedGradScaler()
+                else:
+                    self.scaler = torch.cuda.amp.GradScaler()
             else:
                 if not is_apex_available():
                     raise ImportError(
@@ -420,6 +425,13 @@ class Trainer:
                     )
                 self.use_apex = True
 
+        # FP16 + model parallelism in SageMaker: gradient clipping does not work for now so we raise a helpful error.
+        if is_sagemaker_mp_enabled() and self.use_amp and args.max_grad_norm is not None and args.max_grad_norm > 0:
+            raise ValueError(
+                "SageMaker Model Parallelism in mixed precision mode does not support gradient clipping yet. Pass "
+                "along 'max_grad_norm': 0 in your hyperparameters."
+            )
+
         # Label smoothing
         if self.args.label_smoothing_factor != 0:
             self.label_smoother = LabelSmoother(epsilon=self.args.label_smoothing_factor)
@@ -1607,7 +1619,8 @@ class Trainer:
         inputs = self._prepare_inputs(inputs)
 
         if is_sagemaker_mp_enabled():
-            loss_mb = smp_forward_backward(model, inputs, self.args.gradient_accumulation_steps)
+            scaler = self.scaler if self.use_amp else None
+            loss_mb = smp_forward_backward(model, inputs, self.args.gradient_accumulation_steps, scaler=scaler)
             return loss_mb.reduce_mean().detach().to(self.args.device)
 
         if self.use_amp:

src/transformers/trainer_pt_utils.py

@@ -974,10 +974,15 @@ if is_sagemaker_mp_enabled():
     import smdistributed.modelparallel.torch as smp
 
     @smp.step()
-    def smp_forward_backward(model, inputs, gradient_accumulation_steps=1):
+    def smp_forward_backward(model, inputs, gradient_accumulation_steps=1, scaler=None):
+        with torch.cuda.amp.autocast(enabled=(scaler is not None)):
             outputs = model(**inputs)
+
         loss = outputs["loss"] if isinstance(outputs, dict) else outputs[0]
         loss /= gradient_accumulation_steps
+        if scaler is not None:
+            loss = scaler.scale(loss).squeeze()
+
         model.backward(loss)
         return loss