fix simota candidate_topk (#6979)

ppdet/modeling/assigners/simota_assigner.py

@@ -115,7 +115,10 @@ class SimOTAAssigner(object):
     def dynamic_k_matching(self, cost_matrix, pairwise_ious, num_gt):
         match_matrix = np.zeros_like(cost_matrix.numpy())
         # select candidate topk ious for dynamic-k calculation
-        topk_ious, _ = paddle.topk(pairwise_ious, self.candidate_topk, axis=0)
+        topk_ious, _ = paddle.topk(
+            pairwise_ious,
+            min(self.candidate_topk, pairwise_ious.shape[0]),
+            axis=0)
         # calculate dynamic k for each gt
         dynamic_ks = paddle.clip(topk_ious.sum(0).cast('int'), min=1)
         for gt_idx in range(num_gt):