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

utils/general.py

@@ -181,11 +181,11 @@ def check_requirements(requirements='requirements.txt', exclude=()):
         print(emojis(s))  # emoji-safe
 
 
-def check_img_size(img_size, s=32):
+def check_img_size(img_size, s=32, floor=0):
     # Verify img_size is a multiple of stride s
-    new_size = make_divisible(img_size, int(s))  # ceil gs-multiple
+    new_size = max(make_divisible(img_size, int(s)), floor)  # ceil gs-multiple
     if new_size != img_size:
-        print('WARNING: --img-size %g must be multiple of max stride %g, updating to %g' % (img_size, s, new_size))
+        print(f'WARNING: --img-size {img_size} must be multiple of max stride {s}, updating to {new_size}')
     return new_size