Scatter region when creating tables is not work

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. set global tidb_scatter_region = 1;
2. create table without pre-split option in a new session.
3. grep logs of pd leader to find 'add operator' and then 'mv light peer'.

2. What did you expect to see? (Required)

Get the operator log.

3. What did you see instead (Required)

No this operator log.

4. What is your TiDB version? (Required)

c9af430

[jackysp]

This pr #20980 introduced it. It used the batch API ScatterRegions instead of ScatterRegion, but the batched API doesn't have the same behavior as the single API except it will scatter multi-regions. It is not easy to use the single API directly, because pd doesn't have a single API with an option, which is introduced in #19844 . #19844 is an optimization that could not be reverted directly.
PTAL @Yisaer @JmPotato

[wjhuang2016]

It seems that the basic test is missing.

[nolouch]

fixed.

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.