ddsp.spectral_ops.reset_crepe() should be documented

[turian]

In calling the model multiple times to create audio longer than 4 seconds, I ran into a GPU OOM issue caused by crepe.

ddsp.spectral_ops.reset_crepe() is used in the notebooks, but is not really documented.

It turns out that if you call ddsp.spectral_ops.reset_crepe() repeatedly (once before every model call) that leaks memory. This method should only be called once.

PROPOSED RESOLUTION:

Add code comment to ddsp.spectral_ops.reset_crepe() describing that it should only be called once per session AND that it leaks memory if called repeatedly (instead of just freeing its memory). Each time a colab uses this command, also document there this peculiarity.

Lastly, but not required, it would be very helpful that the timbe transfer colab actually works on the full audio file, instead of 4 seconds.

[turian]

What also should be described is, if I load a new DDSP model, do I need to re-initialize crepe or not?
It would also just be great to fix the underlying leak so repeatedly calling crepe wouldn't be so fraught with peril