Add flag to ignore ffmpeg in python build. (#6199)

* Add flag to ignore ffmpeg in python build.

* Update setup.py to avoid potential collisions with other projects that are using FFMPEG
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

setup.py

@@ -356,6 +356,8 @@ def get_extensions():
             print("Error fetching ffmpeg version, ignoring ffmpeg.")
             has_ffmpeg = False
 
+    use_ffmpeg = os.getenv("TORCHVISION_USE_FFMPEG", "1") == "1"
+    has_ffmpeg = has_ffmpeg and use_ffmpeg
     print(f"FFmpeg found: {has_ffmpeg}")
 
     if has_ffmpeg: