fix double zero_grad call messing up gradient accumulation (#11217)

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

ultralytics/engine/trainer.py

@@ -329,6 +329,7 @@ class BaseTrainer:
             base_idx = (self.epochs - self.args.close_mosaic) * nb
             self.plot_idx.extend([base_idx, base_idx + 1, base_idx + 2])
         epoch = self.start_epoch
+        self.optimizer.zero_grad()  # zero any resumed gradients to ensure stability on train start
         while True:
             self.epoch = epoch
             self.run_callbacks("on_train_epoch_start")
@@ -349,7 +350,6 @@ class BaseTrainer:
                 LOGGER.info(self.progress_string())
                 pbar = TQDM(enumerate(self.train_loader), total=nb)
             self.tloss = None
-            self.optimizer.zero_grad()
             for i, batch in pbar:
                 self.run_callbacks("on_train_batch_start")
                 # Warmup