[fbsync] [references/classification] Adding gradient clipping (#4824)

Summary:
* [references] Adding gradient clipping

* ufmt formatting

* remove apex code

* resolve naming issue

Reviewed By: kazhang

Differential Revision: D32216659

fbshipit-source-id: 9c5ffb102fa5fd9861ae5ba0c44052920c34ebaf

references/classification/train.py

@@ -42,6 +42,10 @@ def train_one_epoch(model, criterion, optimizer, data_loader, device, epoch, arg
         else:
             loss = criterion(output, target)
             loss.backward()
+
+        if args.clip_grad_norm is not None:
+            nn.utils.clip_grad_norm_(utils.get_optimizer_params(optimizer), args.clip_grad_norm)
+
         optimizer.step()
 
         if model_ema and i % args.model_ema_steps == 0:
@@ -472,6 +476,7 @@ def get_args_parser(add_help=True):
     parser.add_argument(
         "--train-crop-size", default=224, type=int, help="the random crop size used for training (default: 224)"
     )
+    parser.add_argument("--clip-grad-norm", default=None, type=float, help="the maximum gradient norm (default None)")
 
     # Prototype models only
     parser.add_argument("--weights", default=None, type=str, help="the weights enum name to load")

references/classification/utils.py

@@ -409,3 +409,11 @@ def reduce_across_processes(val):
     dist.barrier()
     dist.all_reduce(t)
     return t
+
+
+def get_optimizer_params(optimizer):
+    """Generator to iterate over all parameters in the optimizer param_groups."""
+
+    for group in optimizer.param_groups:
+        for p in group["params"]:
+            yield p