rust: support audio (#4566)
rust: support audio (#4566)

Summary:
We now support TF 1.x and TF 2.x audio summaries. This includes TF 2.x
audio summaries with labels; the labels will be stripped. This means
that the audio dashboard now works under `--load_fast`.

Test Plan:
Unit tests should suffice. As an end-to-end test, checked the audio
dashboard against TF 2.x audio summaries both with and without labels
(i.e., of shape `[k, 2]` and of shape `[k]`), and against [TF 1.x data].

[TF 1.x data]: https://gist.github.com/wchargin/d9c415fb609119824919de8391d0b8c4

wchargin-branch: rust-audio

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard