Incremental compilation of the C code

[LaurentMazare]

It seems that the C wrappers get recompiled on pretty much any change. Using ccache makes it a bit better but we should try to fix this as it makes the development loop a bit slow.
A possibility could be to have a subcrate torch-sys which contains the C wrappers so that the C code only gets recompiled when parts of this new crate are modified.

[ehsanmok]

I think sccache is the way to go and can be invoked in the future CI.

[LaurentMazare]

Nice, thanks for pointing this out. This will certainly help on the rust side of things too - I also went ahead and split some bits in a second torch-sys crate so slow compilation of the C++ code is not an issue anymore.