[FLINK-29323] Refine Transformer for VectorAssembler

[weibozhao]

What is the purpose of the change

Refine Transformer for VectorAssembler in Flink ML.

Brief change log

Added a new API(setSizes()) for VectorAssembler transformer.

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): (yes) Does this pull request introduce a new feature? (yes) If yes, how is the feature documented? (Java doc)

[yunfengzhou-hub]

Hi @weibozhao , could you please refine the description and JavaDocs of this PR, illustrating why do we need to add the size parameter to VectorAssembler? What additional functionalities has this parameter support, and how is this parameter compatible with the original public API?

[zhipeng93]

Thanks for the update. LGTM.