Add tfio.experimental.audio.decode_mp3 support

[yongtang]

This PR

• adds tfio.experimental.audio.decode_mp3 support,
• Update minimp3 to 55da78c
• Merge audio and video into one library, as minimp4 serves both audio and video
• This PR also adds encode_mp3, through lame on Linux (macOS and Windows are not supported).

This PR closes #815

/cc @jjedele @faroit

Signed-off-by: Yong Tang yong.tang.github@outlook.com

[jjedele]

@yongtang Looks great, thx for taking over!

[jjedele]

I think the only thing left in #815 is this function to ensure a fixed output shape which we talked about (

io/tensorflow_io/core/python/ops/audio_ops.py

Lines 75 to 100 in cba8bf7

	def _fix_shape(data, desired_channels, desired_samples):
	"""Fix shape of decoded audio to desired channels and samples."""
	org_shape = tf.shape(data)
	org_samples = org_shape[1]
	if desired_samples >= 0:
	# truncate if necessary
	# ignored if org_samples <= desired_samples
	data = data[:, :desired_samples]
	# pad if necessary
	right_pad = desired_samples - org_samples
	if right_pad > 0:
	data = tf.pad(data, [[0, 0], [0, right_pad]], "CONSTANT")
	if desired_channels >= 0:
	out_samples = desired_samples if desired_samples >= 0 else org_samples
	# truncate if necessary
	# ignored if org_channels <= desired_channels
	data = data[:desired_channels, :]
	# convert to mono to stereo if necessary
	data = tf.broadcast_to(data, [desired_channels, out_samples])
	return data

).

Not sure if it's helpful/general to add this here of it's simply enough to write on your own when you need it.

[yongtang]

@jjedele As we discussed I think the def _fix_shape could be a separate API, something like tfio.experimental.audio.normalize. Would you like to work on it?