Fix diagonal matrix check in operations.laplacian.symmetric_normalisation

[smoia]

Closes #

Proposed Changes

Change Type

• bugfix (+0.0.1)
• minor (+0.1.0)
• major (+1.0.0)
• refactoring (no version update)
• test (no version update)
• infrastructure (no version update)
• documentation (no version update)
• other

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[smoia]

🚀 PR was released in 0.9.0 🚀