[Example][Bugfix] Fix dimenet example

[chang-l]

Description

To fix #4204, the crash due to in-place operations for one variable needed for backward propagation.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change
• Related issue is referred in this PR
• If the PR is for a new model/paper, I've updated the example index here.

Changes

Disable grad calculation when reset frequencies.

[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: 1ec024a

Build ID: 1

Status: ✅ CI test succeeded

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[dgl-bot]

Commit ID: 82e4e86

Build ID: 2

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[mufeili]

Resolved.

[dgl-bot]

Commit ID: 934f894

Build ID: 3

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[dgl-bot]

Commit ID: c9fe668

Build ID: 4

Status: ✅ CI test succeeded

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