Reduce warmup epochs from 5 to 1 when finetuning on custom data. (#418)

exps/example/yolox_voc/yolox_voc_s.py

@@ -14,6 +14,7 @@ class Exp(MyExp):
         self.num_classes = 20
         self.depth = 0.33
         self.width = 0.50
+        self.warmup_epochs = 1
         self.exp_name = os.path.split(os.path.realpath(__file__))[1].split(".")[0]
 
     def get_data_loader(self, batch_size, is_distributed, no_aug=False, cache_img=False):