Missing nc and names handling in check_dataset() (ultralytics#4066)

utils/general.py

@@ -231,6 +231,9 @@ def check_dataset(data, autodownload=True):
             if data.get(k):  # prepend path
                 data[k] = str(path / data[k]) if isinstance(data[k], str) else [str(path / x) for x in data[k]]
 
+    assert 'nc' in data, "Dataset 'nc' key missing."
+    if 'names' not in data:
+        data['names'] = [str(i) for i in range(data['nc'])]  # assign class names if missing
     train, val, test, s = [data.get(x) for x in ('train', 'val', 'test', 'download')]
     if val:
         val = [Path(x).resolve() for x in (val if isinstance(val, list) else [val])]  # val path