Remove cv2 from import

Signed-off-by: Jael Gu <mengjia.gu@zilliz.com>

tn.py

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-import cv2
+
 import numpy as np
 import networkx as nx
 from networkx.algorithms.dag import dag_longest_path