Minor build tweaks

[jwallwork23]

This PR:

• Gives some information on additional build dependencies in the README. In particular that torchvision should be installed at the same time as torch.
• Drop FATAL_ERROR from CMake versioning, which is ignored.

[ElliottKasoar]

Do you think it might be worth bumping the minimum CMake version to 3.18, as they have in PyTorch?

I think 3.15 still works in most cases, but CMake 3.18 was the first version to include support for any compiler, enabling CMAKE_CUDA_STANDARD 17 support (it also includes potentially useful things like CMAKE_CUDA_ARCHITECTURES), so it may be safer, although C++17 is already in the FTorch requirements separately.

[jatkinson1000]

PyTorch generally suggests installing torch, torchvision, and torchaudio as a oneshot.
Whilst looking at the conda side of things I feel we do not need torchaudio and torchvision is only a dependency for resnet in example 2, though should check this to be absolutely sure.

I'm still on the fence about FATAL ERROR and whether, given it has no effect beyond CMake >= 2.6, it should be removed.
The docs state that it "should" be included in case anyone tries building with a version lower than 2.6 (unlikely but a possibility). My reading of the CMake docs, though I have not tried doing this, is that if someone tried to build FTorch using an older version of CMake it would not raise an error about the version and instead fail elsewhere in the script.

[henryiii]

I haven't seen CMake <2.8 in many years. CMake 2.6 came out in 2008. I would not worry about what happens if someone tries to run with 2.6; nothing else will work either, it likely wouldn't even run on modern systems.

This is the equivalent of worrying about what error message is printed if a user runs Python 2.5. (Python 2.6 and CMake 2.6 came out the same year).

[matthewfeickert]

(I deleted my previous message as I now understand you were concerned about the effect of CMake v2.x which would print message but keep trying to build, ignoring the cmake_minimum_required minimum version, instead of failing out before the build. Sorry for misunderstanding.)

I agree with Henry that I wouldn't worry about people trying to build with CMake v2.X. Do you know of people you work with that are still using CMake v2?

[jatkinson1000]

Thanks Both, and @jwallwork23
In that case I'll trust your judgment, and I also did a quick scan of linux distros and their requirements and concluded anyone using anything old enough to come with CMake 2.x probably has bigger concerns!!

So I'm happy to approve this for merge.

[jwallwork23]

Do you think it might be worth bumping the minimum CMake version to 3.18, as they have in PyTorch?

I think 3.15 still works in most cases, but CMake 3.18 was the first version to include support for any compiler, enabling CMAKE_CUDA_STANDARD 17 support (it also includes potentially useful things like CMAKE_CUDA_ARCHITECTURES), so it may be safer, although C++17 is already in the FTorch requirements separately.

@ElliottKasoar Thanks for the suggestion. I opened this as a new issue: #234.