Summary:
Split from https://github.com/pytorch/audio/issues/2164
Add new test assets. Adding this commit separately so that
this commit message about the origin of the file is easier to find.

The original video is in public domain par
- https://svs.gsfc.nasa.gov/13013
- https://www.nasa.gov/multimedia/guidelines/index.html
(The YouTube page directly says so)
- https://www.youtube.com/watch?v=6zNsc0e3Zns

So, the video is modified to fit the needs for testing.
1. multiple audio/video streams
2. Non-audio/video (subtitle) streams
3. Different FPS and sampling rate
4. Ones without audio and video.

```
#!/usr/bin/env bash
original=https://svs.gsfc.nasa.gov/vis/a010000/a013000/a013013/NASAs_Most_Scientifically_Complex_Space_Observatory_Requires_Precision-MP4.mp4
subtitle=https://svs.gsfc.nasa.gov/vis/a010000/a013000/a013013/NASAs_Most_Scientifically_Complex_Space_Observatory_Requires_Precision-SRT-CC.en_US.srt

# Fetch the original video, embed the subtitle
ffmpeg -i "${original}" -i "${subtitle}" -c:v copy -c:a copy -c:s mov_text -metadata:s:2 language=eng original.mp4 -y

# Extract, rescale video and resample audio
ffmpeg -i original.mp4 -ss 29 -to 42 -c:s copy -vf scale=480:270 -af aresample=16000 tmp1.mp4 -y
ffmpeg -i original.mp4 -ss 29 -to 42 -c:s copy -vf scale=320:180 -r 25 -af aresample=8000  tmp2.mp4 -y

# Merge them, retaining all the streams (6 in total)
ffmpeg -i tmp2.mp4 -i tmp1.mp4 -map 0 -map 1 -c:s copy nasa_13013.mp4 -y

# Make versions without audio / video
ffmpeg -i tmp2.mp4 -c copy -vn nasa_13013_no_video.mp4 -y
ffmpeg -i tmp2.mp4 -c copy -an nasa_13013_no_video.mp4 -y
```

Pull Request resolved: https://github.com/pytorch/audio/pull/2167

Reviewed By: carolineechen

Differential Revision: D33712954

Pulled By: mthrok

fbshipit-source-id: b7cfc1358043a4abd1c0b416e8a8fb0039867211