support amp training for segmention models (#4994)

* support amp training for segmention models

* fix lint
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

references/segmentation/train.py

@@ -72,19 +72,25 @@ def evaluate(model, data_loader, device, num_classes):
     return confmat
 
 
-def train_one_epoch(model, criterion, optimizer, data_loader, lr_scheduler, device, epoch, print_freq):
+def train_one_epoch(model, criterion, optimizer, data_loader, lr_scheduler, device, epoch, print_freq, scaler=None):
     model.train()
     metric_logger = utils.MetricLogger(delimiter="  ")
     metric_logger.add_meter("lr", utils.SmoothedValue(window_size=1, fmt="{value}"))
     header = f"Epoch: [{epoch}]"
     for image, target in metric_logger.log_every(data_loader, print_freq, header):
         image, target = image.to(device), target.to(device)
-        output = model(image)
-        loss = criterion(output, target)
+        with torch.cuda.amp.autocast(enabled=scaler is not None):
+            output = model(image)
+            loss = criterion(output, target)
 
         optimizer.zero_grad()
-        loss.backward()
-        optimizer.step()
+        if scaler is not None:
+            scaler.scale(loss).backward()
+            scaler.step(optimizer)
+            scaler.update()
+        else:
+            loss.backward()
+            optimizer.step()
 
         lr_scheduler.step()
 
@@ -153,6 +159,8 @@ def main(args):
         params_to_optimize.append({"params": params, "lr": args.lr * 10})
     optimizer = torch.optim.SGD(params_to_optimize, lr=args.lr, momentum=args.momentum, weight_decay=args.weight_decay)
 
+    scaler = torch.cuda.amp.GradScaler() if args.amp else None
+
     iters_per_epoch = len(data_loader)
     main_lr_scheduler = torch.optim.lr_scheduler.LambdaLR(
         optimizer, lambda x: (1 - x / (iters_per_epoch * (args.epochs - args.lr_warmup_epochs))) ** 0.9
@@ -186,6 +194,8 @@ def main(args):
             optimizer.load_state_dict(checkpoint["optimizer"])
             lr_scheduler.load_state_dict(checkpoint["lr_scheduler"])
             args.start_epoch = checkpoint["epoch"] + 1
+            if args.amp:
+                scaler.load_state_dict(checkpoint["scaler"])
 
     if args.test_only:
         confmat = evaluate(model, data_loader_test, device=device, num_classes=num_classes)
@@ -196,7 +206,7 @@ def main(args):
     for epoch in range(args.start_epoch, args.epochs):
         if args.distributed:
             train_sampler.set_epoch(epoch)
-        train_one_epoch(model, criterion, optimizer, data_loader, lr_scheduler, device, epoch, args.print_freq)
+        train_one_epoch(model, criterion, optimizer, data_loader, lr_scheduler, device, epoch, args.print_freq, scaler)
         confmat = evaluate(model, data_loader_test, device=device, num_classes=num_classes)
         print(confmat)
         checkpoint = {
@@ -206,6 +216,8 @@ def main(args):
             "epoch": epoch,
             "args": args,
         }
+        if args.amp:
+            checkpoint["scaler"] = scaler.state_dict()
         utils.save_on_master(checkpoint, os.path.join(args.output_dir, f"model_{epoch}.pth"))
         utils.save_on_master(checkpoint, os.path.join(args.output_dir, "checkpoint.pth"))
 
@@ -269,6 +281,9 @@ def get_args_parser(add_help=True):
     # Prototype models only
     parser.add_argument("--weights", default=None, type=str, help="the weights enum name to load")
 
+    # Mixed precision training parameters
+    parser.add_argument("--amp", action="store_true", help="Use torch.cuda.amp for mixed precision training")
+
     return parser