Fix attribute keyword typo in save function

[joeloskarsson]

Describe your changes

When the wmg.save.to_pyg function is used with the list_from_attribute argument it gives the error:

TypeError: split_graph_by_edge_attribute() got an unexpected keyword argument 'attribute'

This is due to a typo where attribute / attr is mixed up when calling split_graph_by_edge_attribute. This PR fixes this bug and adds a test to make sure saving works with this argument.

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 Documentation (Addition or improvements to documentation)

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[sadamov]

Thanks for fixing this, ready to merge from my side.

[joeloskarsson]

Since this is just a bugfix I'll go ahead and merge it