Add train.py `--img-size floor (#4099)

train.py

@@ -207,7 +207,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
     # Image sizes
     gs = max(int(model.stride.max()), 32)  # grid size (max stride)
     nl = model.model[-1].nl  # number of detection layers (used for scaling hyp['obj'])
-    imgsz = check_img_size(opt.imgsz, gs)  # verify imgsz is gs-multiple
+    imgsz = check_img_size(opt.imgsz, gs, floor=gs * 2)  # verify imgsz is gs-multiple
 
     # DP mode
     if cuda and RANK == -1 and torch.cuda.device_count() > 1: