Call the function Stop when custom op released

[wkcn]

Description

Hi there,
As title, it may fix #18789

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

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[wkcn]

In src/initialize.cc, LibraryInitializer::atfork_prepare() calls CustomOperator::Get()->Stop();.