fix dim check of class center sample (#35733)

python/paddle/nn/functional/common.py

@@ -1701,14 +1701,14 @@ def class_center_sample(label, num_classes, num_samples, group=None):
     label_size = 1
     for dim in list(label.shape):
         label_size *= dim
-    if label_size < 1:
+    if label_size != -1 and label_size < 1:
         raise ValueError('Expected label_size > 0 \
-             (got label_size{})'.format(label_size))
+             (got label_size: {})'.format(label_size))
 
     label_dims = len(list(label.shape))
     if label_dims != 1:
         raise ValueError('Expected label_dims == 1 \
-             (got label_dims{})'.format(label_dims))
+             (got label_dims: {})'.format(label_dims))
 
     seed = None
     if (seed is None or seed == 0) and default_main_program().random_seed != 0: