vendoring and wrapping torchvision, torchtext and torchaudio

[mratsim]

Unlike regular PyTorch/LibTorch, torchvision, torchtext and torchaudio do not have a nice prebuilt executable that we can download.

So we need to either:

• get the library from conda or pip and discarding the Python part
• build the library ourselves on install

Note: we can skip the hell that is CMake and directly use Nim as a C++ build system

For example, for the Arcade learning Environment (Atari games emulator for Reinforcement Learning):

• Original code: https://github.com/mgbellemare/Arcade-Learning-Environment
• Nim build system: https://github.com/numforge/agent-smith/blob/a2d9251/third_party/ale_build.nim#L40-L83
• Bindings is independent: https://github.com/numforge/agent-smith/blob/a2d9251/third_party/ale_wrap.nim

Relevant thread: pytorch/vision#2692 (Using torchvision C++ API without installing python)