Linear LR scheduler option (ultralytics#2150)

* Linear LR scheduler option

* Update train.py

train.py

@@ -120,7 +120,10 @@ def train(hyp, opt, device, tb_writer=None, wandb=None):
 
     # Scheduler https://arxiv.org/pdf/1812.01187.pdf
     # https://pytorch.org/docs/stable/_modules/torch/optim/lr_scheduler.html#OneCycleLR
-    lf = one_cycle(1, hyp['lrf'], epochs)  # cosine 1->hyp['lrf']
+    if opt.linear_lr:
+        lf = lambda x: (1 - x / (epochs - 1)) * (1.0 - hyp['lrf']) + hyp['lrf']  # linear
+    else:
+        lf = one_cycle(1, hyp['lrf'], epochs)  # cosine 1->hyp['lrf']
     scheduler = lr_scheduler.LambdaLR(optimizer, lr_lambda=lf)
     # plot_lr_scheduler(optimizer, scheduler, epochs)
 
@@ -464,6 +467,7 @@ def train(hyp, opt, device, tb_writer=None, wandb=None):
     parser.add_argument('--name', default='exp', help='save to project/name')
     parser.add_argument('--exist-ok', action='store_true', help='existing project/name ok, do not increment')
     parser.add_argument('--quad', action='store_true', help='quad dataloader')
+    parser.add_argument('--linear-lr', action='store_true', help='linear LR')
     opt = parser.parse_args()
 
     # Set DDP variables