Add type hints in mmcv/ops/pixel_group.py (#2016)

* add typehint in mmcv/ops/pixel_group.py

* fix indent

* fix indent

* improve format
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

mmcv/ops/pixel_group.py

 # Copyright (c) OpenMMLab. All rights reserved.
+from typing import List, Union
+
 import numpy as np
 import torch
+from torch import Tensor
 
 from ..utils import ext_loader
 
 ext_module = ext_loader.load_ext('_ext', ['pixel_group'])
 
 
-def pixel_group(score, mask, embedding, kernel_label, kernel_contour,
-                kernel_region_num, distance_threshold):
+def pixel_group(
+    score: Union[np.ndarray, Tensor],
+    mask: Union[np.ndarray, Tensor],
+    embedding: Union[np.ndarray, Tensor],
+    kernel_label: Union[np.ndarray, Tensor],
+    kernel_contour: Union[np.ndarray, Tensor],
+    kernel_region_num: int,
+    distance_threshold: float,
+) -> List[List[float]]:
     """Group pixels into text instances, which is widely used text detection
     methods.