set stream frame_rate to fix tests

Author: Dan-Flores

src/torchcodec/_core/Encoder.cpp

@@ -833,6 +833,10 @@ void VideoEncoder::initializeEncoder(
 
   // Set the stream time base to encode correct frame timestamps
   avStream_->time_base = avCodecContext_->time_base;
+  // Set the stream frame rate to store correct frame durations for some
+  // containers (webm, mkv)
+  avStream_->r_frame_rate = avCodecContext_->framerate;
+
   status = avcodec_parameters_from_context(
       avStream_->codecpar, avCodecContext_.get());
   TORCH_CHECK(

test/test_encoders.py

@@ -1047,18 +1047,16 @@ def test_video_encoder_against_ffmpeg_cli(
 
         # Check that video metadata is the same
         if method == "to_file":
-            # mkv container format handles frame_rate differently, so we skip it here
-            if format != "mkv":
-                fields = ["duration", "duration_ts", "r_frame_rate", "nb_frames"]
-                ffmpeg_metadata = self._get_video_metadata(
-                    ffmpeg_encoded_path,
-                    fields=fields,
-                )
-                encoder_metadata = self._get_video_metadata(
-                    encoder_output_path,
-                    fields=fields,
-                )
-                assert ffmpeg_metadata == encoder_metadata
+            fields = ["duration", "duration_ts", "r_frame_rate", "nb_frames"]
+            ffmpeg_metadata = self._get_video_metadata(
+                ffmpeg_encoded_path,
+                fields=fields,
+            )
+            encoder_metadata = self._get_video_metadata(
+                encoder_output_path,
+                fields=fields,
+            )
+            assert ffmpeg_metadata == encoder_metadata
 
             # Check that frame timestamps and duration are the same
             ffmpeg_frames_info = self._get_frames_info(ffmpeg_encoded_path)