Expose num_workers in VideoClips

[ekosman]

I exposed num_workers in videoClips object because using the original value caused errors in my system

[bjuncek]

Hi @ekosman, thank you very much, but I think (and @fmassa might confirm) that we're moving towards the custom C++ video reader in the followup release following #1303.

Having said that, this looks harmless enough to mere regardless

[ekosman]

Ok thank you @bjuncek, although we discussed this in #1307 but a new video reader sounds super

[bjuncek]

@ekosman fair point - I didn't realize that C++ and PyAv readers are going to co-exist for a while. I call this ok to merge then :)

[fmassa]

LGTM, thanks a lot!

[fmassa]

BTW, can you add some documentation explaining this argument? We didn't add it to _precomputed_metadata because it's an internal argument, but I think num_workers should be documented.

[ekosman]

@fmassa Yes, I'm adding documentation for this parameter. btw, I propose using 0 as default value, so the internal data will be loaded in the main process and it will be more consistent with DataLoader (this is the default value for DataLoader)

[fmassa]

Sounds good using 0 as a default