Update minimum stride to 32 (#2266)

train.py

@@ -161,7 +161,7 @@ def train(hyp, opt, device, tb_writer=None, wandb=None):
         del ckpt, state_dict
 
     # Image sizes
-    gs = int(model.stride.max())  # grid size (max stride)
+    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, imgsz_test = [check_img_size(x, gs) for x in opt.img_size]  # verify imgsz are gs-multiples