[Enhancement] Add type annotations for mmcv/visualization (#1947)

* Add type hints

* Replace mmcv is_str by isinstance str

* Add type hints in optflow.py

* Update mmcv/visualization/optflow.py
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Update mmcv/visualization/color.py
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

* Fix tuple hints
Co-authored-by: Zaida Zhou <58739961+zhouzaida@users.noreply.github.com>

mmcv/visualization/color.py

 # Copyright (c) OpenMMLab. All rights reserved.
 from enum import Enum
+from typing import Union
 
 import numpy as np
 
@@ -21,7 +22,7 @@ class Color(Enum):
     black = (0, 0, 0)
 
 
-def color_val(color):
+def color_val(color: Union[Color, str, tuple, int, np.ndarray]) -> tuple:
     """Convert various input to color tuples.
 
     Args:
@@ -31,7 +32,7 @@ def color_val(color):
         tuple[int]: A tuple of 3 integers indicating BGR channels.
     """
     if is_str(color):
-        return Color[color].value
+        return Color[color].value  # type: ignore
     elif isinstance(color, Color):
         return color.value
     elif isinstance(color, tuple):

mmcv/visualization/optflow.py

 # Copyright (c) OpenMMLab. All rights reserved.
 from __future__ import division
+from typing import Optional, Union
 
 import numpy as np
 
@@ -8,7 +9,9 @@ from mmcv.video import flowread
 from .image import imshow
 
 
-def flowshow(flow, win_name='', wait_time=0):
+def flowshow(flow: Union[np.ndarray, str],
+             win_name: str = '',
+             wait_time: int = 0) -> None:
     """Show optical flow.
 
     Args:
@@ -21,14 +24,16 @@ def flowshow(flow, win_name='', wait_time=0):
     imshow(rgb2bgr(flow_img), win_name, wait_time)
 
 
-def flow2rgb(flow, color_wheel=None, unknown_thr=1e6):
+def flow2rgb(flow: np.ndarray,
+             color_wheel: Optional[np.ndarray] = None,
+             unknown_thr: float = 1e6) -> np.ndarray:
     """Convert flow map to RGB image.
 
     Args:
         flow (ndarray): Array of optical flow.
         color_wheel (ndarray or None): Color wheel used to map flow field to
             RGB colorspace. Default color wheel will be used if not specified.
-        unknown_thr (str): Values above this threshold will be marked as
+        unknown_thr (float): Values above this threshold will be marked as
             unknown and thus ignored.
 
     Returns:
@@ -73,7 +78,7 @@ def flow2rgb(flow, color_wheel=None, unknown_thr=1e6):
     return flow_img
 
 
-def make_color_wheel(bins=None):
+def make_color_wheel(bins: Optional[Union[list, tuple]] = None) -> np.ndarray:
     """Build a color wheel.
 
     Args: