✨🕵️ Fix mypy

[mberr]

• Fix an issue which came up after using the most recent mypy version
• Re-use OneOrSequence type annotation consistently in this method
• Re-use upgrade_to_sequence utility
• get rid of custom PyTorch Geometric installation for tests - this should also fix the issue with CI breaking after each torch release; citing from the README

From PyG 2.3 onwards, you can install and use PyG without any external library required except for PyTorch. For this, simply run

pip install torch_geometric

Additional Libraries

If you want to utilize the full set of features from PyG, there exists several additional libraries you may want to install: [...]