[Cherry-picked 2.0.1] Properly set #samples passed to encoder

[mthrok]

Some audio encoders expect specific, exact number of samples described as in AVCodecContext.frame_size.

The AVFrame.nb_samples is set for the frames passed to AVFilterGraph, but frames coming out of the graph do not necessarily have the same numbr of frames.

This causes issues with encoding OPUS.

This commit fixes it by inserting asetnsamples to filter graph if a fixed number of samples is requested.

Followup:

1. issue warning if encoding opus in FFmpeg 4.1
2. check if AAC issue is caused by this
   

   audio/examples/tutorials/streamwriter_basic_tutorial.py

   Lines 506 to 546 in d8a37a2

   	######################################################################
   	# Note on slicing and AAC
   	# ~~~~~~~~~~~~~~~~~~~~~~~
   	#
   	# .. warning::
   	#
   	# FFmpeg's native AAC encoder (which is used by default when
   	# saving video with MP4 format) has a bug that affects the audibility.
   	#
   	# Please refer to the examples bellow.
   	#
   	def test_slice(audio_encoder, slice_size, ext="mp4"):
   	path = get_path(f"slice_{slice_size}.{ext}")
   	s = StreamWriter(dst=path)
   	s.add_audio_stream(SAMPLE_RATE, NUM_CHANNELS, encoder=audio_encoder)
   	with s.open():
   	for start in range(0, NUM_FRAMES, slice_size):
   	end = start + slice_size
   	s.write_audio_chunk(0, WAVEFORM[start:end, ...])
   	return path
   	######################################################################
   	#
   	# This causes some artifacts.
   	# note:
   	# Chrome does not support playing AAC audio directly while Safari does.
   	# Using MP4 container and specifying AAC allows Chrome to play it.
   	Video(test_slice(audio_encoder="aac", slice_size=8000, ext="mp4"), embed=True)
   	######################################################################
   	#
   	# It is more noticeable when using smaller slice.
   	Video(test_slice(audio_encoder="aac", slice_size=512, ext="mp4"), embed=True)
   	######################################################################
   	#
   	# Lame MP3 encoder works fine for the same slice size.
   	Audio(test_slice(audio_encoder="libmp3lame", slice_size=512, ext="mp3"))

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[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D44374668

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D44374668

[facebook-github-bot]

@mthrok has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D44374668

[facebook-github-bot]

@mthrok merged this pull request in d8a37a2.

[github-actions]

Hey @mthrok.
You merged this PR, but labels were not properly added. Please add a primary and secondary label (See https://github.com/pytorch/audio/blob/main/.github/process_commit.py)