make parameter ignore epochs

ignore epochs functionality add to prevent spikes at the beginning when fitness spikes and decreases after.
Discussed at ultralytics#5971

train.py

@@ -367,7 +367,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
 
             # Update best mAP
             fi = fitness(np.array(results).reshape(1, -1))  # weighted combination of [P, R, mAP@.5, mAP@.5-.95]
-            if fi > best_fitness:
+            if fi > best_fitness and epoch >= opt.ignore_epochs:
                 best_fitness = fi
             log_vals = list(mloss) + list(results) + lr
             callbacks.run('on_fit_epoch_end', log_vals, epoch, best_fitness, fi)
@@ -472,6 +472,7 @@ def parse_opt(known=False):
     parser.add_argument('--freeze', type=int, default=0, help='Number of layers to freeze. backbone=10, all=24')
     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')
+    parser.add_argument('--ignore-epochs', type=int, default=0, help='ignore early epochs to be counted as patience')
 
     # Weights & Biases arguments
     parser.add_argument('--entity', default=None, help='W&B: Entity')