[torchcodec]Remove usage of cv2 in smoke_test

Summary: The smoke test relies on cv2. But in conda its not pre-installed. Use torchvision instead.

Reviewed By: NicolasHug

Differential Revision: D58980922

fbshipit-source-id: 7a71d4b555c6f6330af4ec3add207871a7a3bc01

test/decoders/manual_smoke_test.py

@@ -1,7 +1,7 @@
 import os
 
-import cv2
 import torchcodec
+from torchvision.io.image import write_png
 
 decoder = torchcodec.decoders._core.create_from_file(
     os.path.dirname(__file__) + "/../resources/nasa_13013.mp4"
@@ -10,5 +10,5 @@
 frame = torchcodec.decoders._core.get_frame_at_index(
     decoder, stream_index=3, frame_index=180
 )
-
-cv2.imwrite("frame180.png", frame.numpy())
+frame = frame.permute(2, 0, 1)
+write_png(frame, "frame180.png")