Correctly update num_select during seg model finetuning

[sungchul2]

Description

When fine-tuning seg-preview model, num_select seems to be set to 200, but it is set to 300 like other detection models.
It is due to that this parameter isn't set in SegmentationTrainConfig, so it isn't updated correctly.
With this fix, num_select is correctly updated to 200.

before	after
Namespace(..., num_select=300, ...)	Namespace(..., num_select=200, ...)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

As adding num_select parameter in SegmentationTrainConfig, the argument in Namespace is correctly updated like above.

Any specific deployment considerations

NA

Docs

• Docs updated? What were the changes:

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[sungchul2]

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