Ease the visualization of latent directions

[woctezuma]

There are two changes:

• a CLI argument to be more generic, which lets us sample more than 3 points, compared to:

https://github.com/rosinality/stylegan2-pytorch/blob/7be6e64fc897c5a1df084c9dcc635479c88b2a2f/apply_factor.py#L61-L83

• a transposition of the grid of results as I think it makes more sense to show one sample per row (instead of per column).

This is what the transposed results look like:

NB: I have submitted a bunch of PR. I have merged all of them in one of my branch for my personal use: https://github.com/woctezuma/stylegan2-pytorch/tree/quality-of-life-merge In case you want to merge most of them, this would make it easier for you. Just let me know and I would submit a PR.