Audio: Fix output shape when start==stop (#664)

Author: NicolasHug

src/torchcodec/_core/SingleStreamDecoder.cpp

@@ -848,13 +848,14 @@ AudioFramesOutput SingleStreamDecoder::getFramesPlayedInRangeAudio(
             std::to_string(*stopSecondsOptional) + ").");
   }
 
+  StreamInfo& streamInfo = streamInfos_[activeStreamIndex_];
+
   if (stopSecondsOptional.has_value() && startSeconds == *stopSecondsOptional) {
     // For consistency with video
-    return AudioFramesOutput{torch::empty({0, 0}), 0.0};
+    int numChannels = getNumChannels(streamInfo.codecContext);
+    return AudioFramesOutput{torch::empty({numChannels, 0}), 0.0};
   }
 
-  StreamInfo& streamInfo = streamInfos_[activeStreamIndex_];
-
   auto startPts = secondsToClosestPts(startSeconds, streamInfo.timeBase);
   if (startPts < streamInfo.lastDecodedAvFramePts +
           streamInfo.lastDecodedAvFrameDuration) {

test/test_decoders.py

@@ -1108,7 +1108,7 @@ def test_not_at_frame_boundaries(self, asset):
     def test_start_equals_stop(self, asset):
         decoder = AudioDecoder(asset.path)
         samples = decoder.get_samples_played_in_range(start_seconds=3, stop_seconds=3)
-        assert samples.data.shape == (0, 0)
+        assert samples.data.shape == (asset.num_channels, 0)
 
     def test_frame_start_is_not_zero(self):
         # For NASA_AUDIO_MP3, the first frame is not at 0, it's at 0.138125.

test/test_ops.py

@@ -782,7 +782,7 @@ def test_decode_start_equal_stop(self, asset):
         frames, pts_seconds = get_frames_by_pts_in_range_audio(
             decoder, start_seconds=1, stop_seconds=1
         )
-        assert frames.shape == (0, 0)
+        assert frames.shape == (asset.num_channels, 0)
         assert pts_seconds == 0
 
     @pytest.mark.parametrize("asset", (NASA_AUDIO, NASA_AUDIO_MP3))