Implementation of Early Stopping for DDP training (ultralytics#8345)

* Implementation of Early Stopping for DDP training

This edit correctly uses the broadcast_object_list() function to send slave processes a boolean so to end the training phase if the variable is True, thus allowing the master process to destroy the process group and terminate.

* Update train.py

* Update train.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Update train.py

* Update train.py

* Update train.py

* Further cleanup

This cleans up the definition of broadcast_list and removes the requirement for clear() afterward.

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

train.py

@@ -294,7 +294,7 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
     results = (0, 0, 0, 0, 0, 0, 0)  # P, R, mAP@.5, mAP@.5-.95, val_loss(box, obj, cls)
     scheduler.last_epoch = start_epoch - 1  # do not move
     scaler = torch.cuda.amp.GradScaler(enabled=amp)
-    stopper = EarlyStopping(patience=opt.patience)
+    stopper, stop = EarlyStopping(patience=opt.patience), False
     compute_loss = ComputeLoss(model)  # init loss class
     callbacks.run('on_train_start')
     LOGGER.info(f'Image sizes {imgsz} train, {imgsz} val\n'
@@ -402,6 +402,7 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
 
             # Update best mAP
             fi = fitness(np.array(results).reshape(1, -1))  # weighted combination of [P, R, mAP@.5, mAP@.5-.95]
+            stop = stopper(epoch=epoch, fitness=fi)  # early stop check
             if fi > best_fitness:
                 best_fitness = fi
             log_vals = list(mloss) + list(results) + lr
@@ -428,19 +429,14 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
                 del ckpt
                 callbacks.run('on_model_save', last, epoch, final_epoch, best_fitness, fi)
 
-            # Stop Single-GPU
-            if RANK == -1 and stopper(epoch=epoch, fitness=fi):
-                break
-
-            # Stop DDP TODO: known issues shttps://github.com/ultralytics/yolov5/pull/4576
-            # stop = stopper(epoch=epoch, fitness=fi)
-            # if RANK == 0:
-            #    dist.broadcast_object_list([stop], 0)  # broadcast 'stop' to all ranks
-
-        # Stop DPP
-        # with torch_distributed_zero_first(RANK):
-        # if stop:
-        #    break  # must break all DDP ranks
+        # EarlyStopping
+        if RANK != -1:  # if DDP training
+            broadcast_list = [stop if RANK == 0 else None]
+            dist.broadcast_object_list(broadcast_list, 0)  # broadcast 'stop' to all ranks
+            if RANK != 0:
+                stop = broadcast_list[0]
+        if stop:
+            break  # must break all DDP ranks
 
         # end epoch ----------------------------------------------------------------------------------------------------
     # end training -----------------------------------------------------------------------------------------------------