Deprecate _transforms_video and _functional_video (#3441)

* deprecate _video modules

* flake8
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

test/test_transforms_video.py

 import torch
-import torchvision.transforms._transforms_video as transforms
 from torchvision.transforms import Compose
 import unittest
 import random
 import numpy as np
+import warnings
 
 try:
     from scipy import stats
@@ -11,6 +11,11 @@ except ImportError:
     stats = None
 
 
+with warnings.catch_warnings(record=True):
+    warnings.simplefilter("always")
+    import torchvision.transforms._transforms_video as transforms
+
+
 class TestVideoTransforms(unittest.TestCase):
 
     def test_random_crop_video(self):

torchvision/transforms/_functional_video.py

 import torch
+import warnings
+
+
+warnings.warn(
+    "The _functional_video module is deprecated. Please use the functional module instead."
+)
 
 
 def _is_tensor_video_clip(clip):

torchvision/transforms/_transforms_video.py

@@ -2,6 +2,7 @@
 
 import numbers
 import random
+import warnings
 
 from torchvision.transforms import (
     RandomCrop,
@@ -21,6 +22,11 @@ __all__ = [
 ]
 
 
+warnings.warn(
+    "The _transforms_video module is deprecated. Please use the transforms module instead."
+)
+
+
 class RandomCropVideo(RandomCrop):
     def __init__(self, size):
         if isinstance(size, numbers.Number):