Deprecate PWC-Net, change env name to 'video_features', update .gitignore

[v-iashin]

convinced myself to deprecate PWC-Net. the implementation of it taxed me for a very long time and people were reporting problems with it #13 #6 #8.

the list goes a long way, to name a few considerations:
a) it requires a separate conda environment,
b) it depends on cupy that is difficult to maintain,
c) current implementation does not support newer GPUs,
d) RAFT is a better alternative,
e) the implementation that I used is licensed as GPL which makes the whole library to be GPL.

overall, it is not worth the effort to maintain it. it's time to move on, I am afraid.

another change that this PR brings is the environment name: torch_zoo --> video_features

TODO:

• remove the figure from the gif, draw io, and from toc in docs (but keep the page)
• run the tests
• fix typos
• fix license
• give a user an error pointing to this pr if they specify pwc as optical flow