import cv2 inside class (#5659)

The line `optional_import("cv2")` causes the security light of my macOS
to light up in green, even for the lowest level command e.g., `import
monai`. I was worried I had a virus until I realised what was causing
it! To avoid worrying other users, I suggest we only import when a user
is using the `VideoDataset`.

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).

Signed-off-by: Richard Brown <33289025+rijobro@users.noreply.github.com>
Signed-off-by: Wenqi Li <wenqil@nvidia.com>
Co-authored-by: Wenqi Li <wenqil@nvidia.com>
Co-authored-by: Wenqi Li <831580+wyli@users.noreply.github.com>

Author: rijobro

monai/data/dataset.py

@@ -1091,16 +1091,14 @@ def is_started(self):
         Check whether the replacement thread is already started.
 
         """
-        if self._replace_mgr is None:
-            return False
-        return self._replace_mgr.is_alive()
+        return False if self._replace_mgr is None else self._replace_mgr.is_alive()
 
     def start(self):
         """
         Start the background thread to replace training items for every epoch.
 
         """
-        if self._replace_mgr is None or not self.is_started():
+        if not self.is_started():
             self._restart()
 
     def _restart(self):
@@ -1141,8 +1139,7 @@ def update_cache(self):
         If the cache has been shutdown before, need to restart the `_replace_mgr` thread.
 
         """
-        if not self._replace_mgr.is_alive():
-            self._restart()
+        self.start()
 
         # make sure update is done
         while not self._try_update_cache():
@@ -1173,7 +1170,7 @@ def shutdown(self):
         # wait until replace mgr is done the current round
         while not self._try_shutdown():
             time.sleep(0.01)
-        self._replace_mgr.join()
+        self._replace_mgr.join(300)
 
     def _replace_cache_thread(self, index: int):
         """

monai/data/video_dataset.py

@@ -20,14 +20,21 @@
 from monai.utils.enums import ColorOrder
 from monai.utils.module import optional_import
 
+__all__ = ["VideoDataset", "VideoFileDataset", "CameraDataset"]
+
 if TYPE_CHECKING:
     import cv2
 
     has_cv2 = True
 else:
-    cv2, has_cv2 = optional_import("cv2")
+    cv2, has_cv2 = None, None
 
-__all__ = ["VideoDataset", "VideoFileDataset", "CameraDataset"]
+
+def import_cv():
+    """Import cv2. Put it inside a function to avoid webcam lights blinking on ``import monai``."""
+    global cv2
+    global has_cv2
+    cv2, has_cv2 = optional_import("cv2")
 
 
 class SuppressStderr:
@@ -50,6 +57,9 @@ def __exit__(self, *_):
 
 
 class VideoDataset:
+    # import inside class to avoid webcam blinking on ``import monai``.
+    import_cv()
+
     def __init__(
         self,
         video_source: Union[str, int],