[BugFix] Fix khop_adj

[rudongyu]

Description

Current khop_adj api wrongly includes a transpose operation when accessing adjacency matrix. Fix the bug.

Checklist

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• Related issue is referred in this PR
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[dgl-bot]

To trigger regression tests:

• @dgl-bot run [instance-type] [which tests] [compare-with-branch];
  For example: @dgl-bot run g4dn.4xlarge all dmlc/master or @dgl-bot run c5.9xlarge kernel,api dmlc/master

[dgl-bot]

Commit ID: 00ab57e

Build ID: 1

Status: ✅ CI test succeeded.

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