Minimal conda environment with cuda

[fwilliams]

This PR adds a minimal conda environment with system packages for cuda and compiler tools which can be used as a starting point for building fvdb (pip install -r env/build_requirements.txt). This acts as a good transition as we move over to pip-only for the release.

[harrism]

How will this be used, exactly? If you have a specific use case, should probably include doc changes in this PR.

Since conda environments are (sadly) not composable, to me this just seems like one more environment configuration we have to maintain.

[fwilliams]

@harrism
this is just to have a minimal environment that sets up cmake, cuda and the right compiler but nothing else
at some point, I imagine we'll have requirements.txt for everything and use pip. If you still want a conda environment for some reason (maybe you want to install your own packages), this gives you one so you can at least do:

conda env create -f env/minimal_environment.yml
conda activate fvdb_min
pip install -r requirements.txt
... # build, test, etc.

[fwilliams]

Let's do the requirements.txt in one big PR and I'll resubmit then. Converting to a draft for now.