Handle vorbis extension in ffmpeg backend

When dealing with vorbis format, FFmpeg expects "ogg" container/extension
with "vorbis" encoder. It does not recognize "vorbis" container/extension.

libsox-based torchaudio I/O used to handle vorbis extension.
This commit adds support to vorbis as extension for those cases with FFmpeg backend.

torchaudio/io/_compat.py

@@ -102,7 +102,8 @@ def load_audio_fileobj(
     format: Optional[str] = None,
     buffer_size: int = 4096,
 ) -> Tuple[torch.Tensor, int]:
-    s = torchaudio.lib._torchaudio_ffmpeg.StreamReaderFileObj(src, format, None, buffer_size)
+    file_format = "ogg" if format == "vorbis" else format
+    s = torchaudio.lib._torchaudio_ffmpeg.StreamReaderFileObj(src, file_format, None, buffer_size)
     sample_rate = int(s.get_src_stream_info(s.find_best_audio_stream()).sample_rate)
     filter = _get_load_filter(frame_offset, num_frames, convert)
     waveform = _load_audio_fileobj(s, filter, channels_first)
@@ -204,25 +205,32 @@ def save_audio(
     bits_per_sample: Optional[int] = None,
     buffer_size: int = 4096,
 ) -> None:
+    ext = None
     if hasattr(uri, "write"):
         if format is None:
             raise RuntimeError("'format' is required when saving to file object.")
     else:
         uri = os.path.normpath(uri)
-    s = StreamWriter(uri, format=format, buffer_size=buffer_size)
-    if format is None:
-        tokens = str(uri).split(".")
-        if len(tokens) > 1:
-            format = tokens[-1].lower()
+        if tokens := str(uri).split(".")[1:]:
+            ext = tokens[-1].lower()
+
+    # FFpmeg does not recognize vorbis extension, while libsox used to do.
+    # For the sake of bakward compatibility, (and the simplicity),
+    # we support the case where users want to do save("foo.vorbis")
+    if (ext == "vorbis" and format is None) or hasattr(uri, "write") and format == "vorbis":
+        file_format = "ogg"
+    else:
+        file_format = format
+    s = StreamWriter(uri, format=file_format, buffer_size=buffer_size)
 
     if channels_first:
         src = src.T
     s.add_audio_stream(
         sample_rate,
         num_channels=src.size(-1),
         format=_get_sample_format(src.dtype),
-        encoder=_get_encoder(src.dtype, format, encoding, bits_per_sample),
-        encoder_format=_get_encoder_format(format, bits_per_sample),
+        encoder=_get_encoder(src.dtype, ext, encoding, bits_per_sample),
+        encoder_format=_get_encoder_format(ext, bits_per_sample),
     )
     with s.open():
         s.write_audio_chunk(0, src)