session: fix dml_batch_size doesn't load the global variable (#24710)

[ti-srebot]

cherry-pick #24710 to release-5.0
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/24731

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---

What problem does this PR solve?

Issue Number: close #24709

Problem Summary:
No TiDBDMLBatchSize in builtinGlobalVariable. So when loading the global variable, it misses TiDBDMLBatchSize.

What is changed and how it works?

Add TiDBDMLBatchSize to builtinGlobalVariable.

How it Works:

Related changes

• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Side effects

Release note

• fix dml_batch_size doesn't load the global variable

[ti-chi-bot]

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[ti-srebot]

/run-all-tests

[ti-srebot]

@wjhuang2016 you're already a collaborator in bot's repo.

[cfzjywxk]

/lgtm

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[xiongjiwei]

lgtm

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• djshow832
• xiongjiwei

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[jackysp]

In TiDB 5.0, the default tidb_dml_batch_size is 0, it will make load data will not split the transaction. But if the user does not care about the atomic of transaction and wants a lower heap memory usage of load data, he will want to set tidb_dml_batch_size not 0.

[djshow832]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 90a45e4

[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.

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