feat: parallel processing.

[ooobsidian]

Parallel processing is used in the generation of speech feature files.

[philipperemy]

@ooobsidian so actually it's already using multi threading to process the audio. Just run the commands and you'll see that all your cores are used. This PR does not seem to be necessary. Let me know!

[ooobsidian]

But I only have one thread in execution, and it's slow.

[philipperemy]

Hum that's weird. When I run the code, mine uses all my 12 cores.

[ooobsidian]

I hope you can merge this PR, as there is only one core in use at my runtime.

[philipperemy]

But let's try to understand first why before I merged. Because in my case if I start the program with multi processes and multi threadings at the same time, it will go bad.

[ooobsidian]

Anyhow, I want make it faster.

[philipperemy]

@ooobsidian sure go ahead with your implementation for now. I'll need to check that in details to understand why we have different behaviors :)

[ooobsidian]

I think it should be running this program at the same time as the other programs, causing the IO to be slow.