Multi-layer capable --freeze argument (ultralytics#6019)

* support specfiy multiple frozen layers

* fix bug

* Cleanup Freeze section

* Cleanup argument

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

train.py

@@ -124,7 +124,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
         model = Model(cfg, ch=3, nc=nc, anchors=hyp.get('anchors')).to(device)  # create
 
     # Freeze
-    freeze = [f'model.{x}.' for x in range(freeze)]  # layers to freeze
+    freeze = [f'model.{x}.' for x in (freeze if isinstance(freeze, list) else range(freeze))]  # layers to freeze
     for k, v in model.named_parameters():
         v.requires_grad = True  # train all layers
         if any(x in k for x in freeze):
@@ -469,7 +469,7 @@ def parse_opt(known=False):
     parser.add_argument('--linear-lr', action='store_true', help='linear LR')
     parser.add_argument('--label-smoothing', type=float, default=0.0, help='Label smoothing epsilon')
     parser.add_argument('--patience', type=int, default=100, help='EarlyStopping patience (epochs without improvement)')
-    parser.add_argument('--freeze', type=int, default=0, help='Number of layers to freeze. backbone=10, all=24')
+    parser.add_argument('--freeze', nargs='+', type=int, default=0, help='Freeze layers: backbone=10, first3=0 1 2')
     parser.add_argument('--save-period', type=int, default=-1, help='Save checkpoint every x epochs (disabled if < 1)')
     parser.add_argument('--local_rank', type=int, default=-1, help='DDP parameter, do not modify')