Patch for compatibility with PyTorch ≥1.9

[khanhvu207]

Hi,

This PR updates the SpectralSteinEstimator for compatibility with modern PyTorch 2.7. In particular, PyTorch ≥ 1.9 deprecates and removes torch.eig. This caused a runtime error when computing eigenvalues of the Gram matrix. The fix:

1. Replace line 78:

eigen_vals, eigen_vecs = torch.eig(Kxx, eigenvectors=True)

with:

eigen_vals, eigen_vecs = torch.linalg.eig(Kxx)
eigen_vals = eigen_vals.real
eigen_vecs = eigen_vecs.real

2. And replace eigen_vals[:, 0] with just eigen_vals.