Paper cuts post `torchsim` migration

[HaoZeke]

Following the merge of #167, we should bring the torchsim integration up to speed with the latest features in metatomic:

• Get the ability to use non_conservative_forces and non_conservative_stress outputs
• Add support for energy uncertainty
• Add support for variants
• Bring back the documentation that was removed during the PR

[HaoZeke]

https://github.com/metatensor/metatomic/pull/167/changes#r2918554531

[CompRhys]

what is the plan to push to pypi such that I can merge TorchSim/torch-sim#510?