[FBcode->GH] Move loading Decoder.so to a separate file to avoid building it internally (#5183)

torchvision/io/__init__.py

@@ -3,10 +3,14 @@ from typing import Any, Dict, Iterator
 import torch
 
 from ..utils import _log_api_usage_once
+
+try:
+    from ._load_gpu_decoder import _HAS_VIDEO_DECODER
+except ModuleNotFoundError:
+    _HAS_VIDEO_DECODER = False
 from ._video_opt import (
     Timebase,
     VideoMetaData,
-    _HAS_VIDEO_DECODER,
     _HAS_VIDEO_OPT,
     _probe_video_from_file,
     _probe_video_from_memory,

torchvision/io/_load_gpu_decoder.py

+from ..extension import _load_library
+
+
+try:
+    _load_library("Decoder")
+    _HAS_VIDEO_DECODER = True
+except (ImportError, OSError):
+    _HAS_VIDEO_DECODER = False

torchvision/io/_video_opt.py

@@ -14,12 +14,6 @@ try:
 except (ImportError, OSError):
     _HAS_VIDEO_OPT = False
 
-try:
-    _load_library("Decoder")
-    _HAS_VIDEO_DECODER = True
-except (ImportError, OSError):
-    _HAS_VIDEO_DECODER = False
-
 default_timebase = Fraction(0, 1)