Add an O(3)-symmetrized atomistic model wrapper

[MichelangeloDomina]

Summary

Adds metatomic.torch.symmetrized_model: wrap an exported AtomisticModel to
average its predictions over O(3) with a finite rotation quadrature, and to
measure how far the underlying model is from being equivariant.

Everything is selected through regular output requests:

Requested output	Result
<name>	O(3) average of the underlying output
o3::variance::<name>	per-sample equivariance variance
o3::character_projection::<name>	squared character-projection contribution per irrep

SymmetrizedModel.wrap() returns a regular exportable AtomisticModel
(metadata, capabilities, neighbor lists, and custom inputs are carried over),
so the wrapped model can be used anywhere the original could, including
TorchScript export.

The conventions (quadrature, variance definition, irrep bookkeeping in
TensorMap keys) are documented in docs/src/torch/reference/symmetrized-model.rst.

Limitations

• the quadrature is exact only up to a maximal angular momentum; convergence
  should be checked by increasing max_o3_lambda_grid;
• CPU and CUDA only, float32/float64;
• explicit TensorBlock gradients are not supported (PyTorch autograd through
  returned values works).

Contributor (creator of pull-request) checklist

• Tests updated (for new features and bugfixes)?
• Documentation updated (for new features)?
• [ ] Issue referenced (for PRs that solve an issue)?

Reviewer checklist

• CHANGELOG updated with public API or any other important changes?