always specify custom value for `sanity_check_paths` in custom easyblock for torchvision

[boegel]

(created using eb --new-pr)

This fixes a problem for the sanity check of PyTorch-bundle which was introduced in #3322:

Sanity check failed: no (non-empty) directory
found at 'bin' in /user/gent/400/vsc40023/eb_scratch/RHEL8/cascadelake-ampere-ib/software/PyTorch-bundle/2.1.2-foss-2023a

The problem is that custom_paths in sanity_check_step is left empty when libjpeg-turbo is used as dependency.

The PythonPackage easyblock avoids that the default bin+lib value for sanity_check_paths is used (by using lib/py%(pyshortver)s/site-packages instead), but it only does this for standalone installations (not for extensions).

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS PyTorch-bundle-2.1.2-foss-2023a.eb
• SUCCESS torchvision-0.14.1-foss-2022b.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total) node4001.donphan.os - Linux RHEL 8.8, x86_64, Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz (cascadelake), 1 x NVIDIA NVIDIA A2, 545.23.08, Python 3.6.8 See https://gist.github.com/boegel/66b48a43a6ca64b01999373bcdba7f4b for a full test report.

[smoors]

Going in, thanks @boegel!