fix duplicated edges in link split for directed tasks

[codecov[bot]]

Codecov Report

Base: 99.52% // Head: 99.52% // Increases project coverage by +0.00% :tada:

Coverage data is based on head (06509b1) compared to base (803483b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #45 +/- ## ======================================= Coverage 99.52% 99.52% ======================================= Files 77 77 Lines 3758 3759 +1 ======================================= + Hits 3740 3741 +1 Misses 18 18 ``` | [Impacted Files](https://codecov.io/gh/SherylHYX/pytorch_geometric_signed_directed/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yixuan+He) | Coverage Δ | | |---|---|---| | [...metric\_signed\_directed/utils/general/link\_split.py](https://codecov.io/gh/SherylHYX/pytorch_geometric_signed_directed/pull/45/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yixuan+He#diff-dG9yY2hfZ2VvbWV0cmljX3NpZ25lZF9kaXJlY3RlZC91dGlscy9nZW5lcmFsL2xpbmtfc3BsaXQucHk=) | `96.31% <100.00%> (+0.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yixuan+He). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yixuan+He)

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