Partial hessian

[Phorbol]

This PR adds inference-time partial Hessian support (ASE FixAtoms-style constraints), plus a convenience API to produce the free-atom 2D Hessian submatrix compatible with ASE vibrations workflows, while preserving backward compatibility with MACE’s existing Hessian output layout.

• Core: partial Hessian computation

  • Extend Hessian routines in [utils.py] to accept an optional indices/hessian_indices argument so only selected atoms’ force components are differentiated (computes only required Hessian rows).
  • Thread hessian_indices through get_outputs and the energy models in [models.py](MACE, ScaleShiftMACE, MACELES).

• ASE calculator: constraints → free indices

  • Update MACECalculator.get_hessian in [mace.py] to derive free_indices via ase.vibrations.data.VibrationsData.indices_from_constraints(atoms) when available, with a FixAtoms fallback.
  • Keep the original return shape (3N, N, 3) for compatibility; rows for fixed atoms are zero-filled, free-atom rows contain the computed partial Hessian rows.

• New API: free-atom 2D Hessian for ASE vib

  • Add MACECalculator.get_hessian_2d_free(atoms) -> (H2d, free_indices) in [mace.py], returning a (3*Nfree, 3*Nfree) Hessian submatrix (from the (3N, N, 3) tensor) plus free_indices.
  • Include a ready-to-run example in the method docstring using VibrationsData.from_2d(...).

• Tests

  • Add/update [test_partial_hessian.py] to verify:
    • fixed-atom Hessian rows are zero,
    • free-atom rows match the full Hessian rows,
    • get_hessian_2d_free matches the corresponding free-atom submatrix extracted from the full Hessian.