CI tests only run on conda

[fmassa]

🚀 Feature

Run CI tests on pip as well.

Motivation

We have traditionally built torchvision via both conda and pip, but we only perform tests on conda.

This has been generally fine because there was nothing which was specific to pip that could affect runtime and wouldn't be catch by a conda test.

This is not true anymore since the addition of libpng / libjpeg / ffmpeg as dependencies to torchvision, which requires different manipulations for pip.

This lack of coverage is illustrated in #2775, where only the conda binaries work, and not the pip packages.

cc @andfoy @seemethere

[mthrok]

but we only perform tests on conda.

This sounds like unit test, but can you clarify which test do are you proposing to run on pip installation?
Typically, unit tests are not designed for installation check, rather it assumes sanity installation.

If you want to check the installation, then vision has already smoke tests, so it seems to me that you can perform the check there too.

[fmassa]

Yes, better smoke tests could also be a possibility. This is the type of smoke test I would add in addition to importing torchvision.

One thing to take into account though is that smoke tests generally only run on a schedule, but while implementing / debugging a new feature that has different handling for pip / conda, having iterative CI signal is valuable.