[FLINK-33968][runtime] Advance the calculation of num of subpartitions to the time of initializing execution job vertex

[wanglijie95]

What is the purpose of the change

Solve FLINK-33968

Verifying this change

This change is already covered by existing tests.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[wanglijie95]

@JunRuiLee @zhuzhurk Could you help to review this PR ? Thanks.

[flinkbot]

CI report:

• 433585f Azure: SUCCESS

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[zhuzhurk]

Thanks for creating this PR. @wanglijie95
LGTM.