Fix graph vertex ordering during topological ordering

[shaahji]

Fix graph vertex ordering during topological ordering

Previous implementation returned vertices in reverse order even when there are no dependencies in the graph. Iterate the vertices in reverse order so input order can be retained.

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• Add unit tests for this change.
• Make sure all tests can pass.
• Update documents if necessary.
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• Is this a user-facing change? If yes, give a description of this change to be included in the release notes.
• Is this PR including examples changes? If yes, please remember to update example documentation in a follow-up PR.

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