provide different env.yml file for local installation without cupy and cudatoolkit

[wikfeldt]

No description provided.

[qianglise]

i still did not get why they have errors about cupy and cudatoolkit. Maybe they are using Windows?
In general, there should not be problems installing them.
In the beginning, I planned to have cupy examples, that is why it is there.
cudatoolkit is needed for numba anyhow, so one should not remove.

[qianglise]

maybe we could make one yml file for cpu, then people could use that to install locally

[wikfeldt]

yeah i think the failure to install cupy/cudatoolkit was on windows. But i agree that having two env.yml files, one for CPU and one for GPU, makes sense.
But we might want to consider larger changes, like splitting the material into CPU and GPU parts. This is for a later discussion