support amp training for detection models (#4933)

* support amp training

* support amp training

* support amp training

* Update references/detection/train.py

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

* Update references/detection/engine.py

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

* fix lint issues

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

Author: xiaohu2015

references/detection/engine.py

@@ -9,7 +9,7 @@
 from coco_utils import get_coco_api_from_dataset
 
 
-def train_one_epoch(model, optimizer, data_loader, device, epoch, print_freq):
+def train_one_epoch(model, optimizer, data_loader, 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:.6f}"))
@@ -27,10 +27,9 @@ def train_one_epoch(model, optimizer, data_loader, device, epoch, print_freq):
     for images, targets in metric_logger.log_every(data_loader, print_freq, header):
         images = list(image.to(device) for image in images)
         targets = [{k: v.to(device) for k, v in t.items()} for t in targets]
-
-        loss_dict = model(images, targets)
-
-        losses = sum(loss for loss in loss_dict.values())
+        with torch.cuda.amp.autocast(enabled=scaler is not None):
+            loss_dict = model(images, targets)
+            losses = sum(loss for loss in loss_dict.values())
 
         # reduce losses over all GPUs for logging purposes
         loss_dict_reduced = utils.reduce_dict(loss_dict)
@@ -44,8 +43,13 @@ def train_one_epoch(model, optimizer, data_loader, device, epoch, print_freq):
             sys.exit(1)
 
         optimizer.zero_grad()
-        losses.backward()
-        optimizer.step()
+        if scaler is not None:
+            scaler.scale(losses).backward()
+            scaler.step(optimizer)
+            scaler.update()
+        else:
+            losses.backward()
+            optimizer.step()
 
         if lr_scheduler is not None:
             lr_scheduler.step()

references/detection/train.py

@@ -144,6 +144,9 @@ 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
 
 
@@ -209,6 +212,8 @@ def main(args):
     params = [p for p in model.parameters() if p.requires_grad]
     optimizer = torch.optim.SGD(params, lr=args.lr, momentum=args.momentum, weight_decay=args.weight_decay)
 
+    scaler = torch.cuda.amp.GradScaler() if args.amp else None
+
     args.lr_scheduler = args.lr_scheduler.lower()
     if args.lr_scheduler == "multisteplr":
         lr_scheduler = torch.optim.lr_scheduler.MultiStepLR(optimizer, milestones=args.lr_steps, gamma=args.lr_gamma)
@@ -225,6 +230,8 @@ 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:
         evaluate(model, data_loader_test, device=device)
@@ -235,7 +242,7 @@ def main(args):
     for epoch in range(args.start_epoch, args.epochs):
         if args.distributed:
             train_sampler.set_epoch(epoch)
-        train_one_epoch(model, optimizer, data_loader, device, epoch, args.print_freq)
+        train_one_epoch(model, optimizer, data_loader, device, epoch, args.print_freq, scaler)
         lr_scheduler.step()
         if args.output_dir:
             checkpoint = {
@@ -245,6 +252,8 @@ def main(args):
                 "args": args,
                 "epoch": epoch,
             }
+            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"))