More flexible build script for Android support

[laptou]

I wanted to be able to bind tch to PyTorch Mobile (libtorch on Android). To this end, I made a few changes to the build script:

• I added environment specific environment variables for giving the path to libtorch (ex.: LIBTORCH_LIB_aarch64-linux-android) so that I can give a path to a different libtorch for each architecture and OS.
• I removed the include and lib from the LIBTORCH_INCLUDE and LIBTORCH_LIB paths, so the user is now supposed to include these in the paths themselves. This is a breaking change, but it was necessary to make things work on Android, because the folders have different names there.
• PyTorch has two possible build for Android, a "lite" version and a normal one. They have different names for the .so file, so I added a LIBTORCH_LITE file to tell the build script which one we are linking against.

I also made some changes to the code, namely support for moving tensors to the Vulkan device. I was disappointed to find out that PyTorch support for Vulkan is pretty half-baked, but still.