Added typing annotations to io/__init__ (#4224)

* style: Added typing annotations

* Specified types for iter and seek.
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>
Co-authored-by: Vasilis Vryniotis <vvryniotis@fb.com>

torchvision/io/__init__.py

 import torch
+from typing import Any, Dict, Iterator
 
 from ._video_opt import (
     Timebase,
@@ -33,13 +34,13 @@ from .image import (
 
 if _HAS_VIDEO_OPT:
 
-    def _has_video_opt():
+    def _has_video_opt() -> bool:
         return True
 
 
 else:
 
-    def _has_video_opt():
+    def _has_video_opt() -> bool:
         return False
 
 
@@ -99,7 +100,7 @@ class VideoReader:
             Currently available options include ``['video', 'audio']``
     """
 
-    def __init__(self, path, stream="video"):
+    def __init__(self, path: str, stream: str = "video") -> None:
         if not _has_video_opt():
             raise RuntimeError(
                 "Not compiled with video_reader support, "
@@ -109,7 +110,7 @@ class VideoReader:
             )
         self._c = torch.classes.torchvision.Video(path, stream)
 
-    def __next__(self):
+    def __next__(self) -> Dict[str, Any]:
         """Decodes and returns the next frame of the current stream.
         Frames are encoded as a dict with mandatory
         data and pts fields, where data is a tensor, and pts is a
@@ -126,10 +127,10 @@ class VideoReader:
             raise StopIteration
         return {"data": frame, "pts": pts}
 
-    def __iter__(self):
+    def __iter__(self) -> Iterator['VideoReader']:
         return self
 
-    def seek(self, time_s: float):
+    def seek(self, time_s: float) -> 'VideoReader':
         """Seek within current stream.
 
         Args:
@@ -144,7 +145,7 @@ class VideoReader:
         self._c.seek(time_s)
         return self
 
-    def get_metadata(self):
+    def get_metadata(self) -> Dict[str, Any]:
         """Returns video metadata
 
         Returns:
@@ -152,7 +153,7 @@ class VideoReader:
         """
         return self._c.get_metadata()
 
-    def set_current_stream(self, stream: str):
+    def set_current_stream(self, stream: str) -> bool:
         """Set current stream.
         Explicitly define the stream we are operating on.