style(batch scheduler): some style improvements

[xxchan]

What's changed and what's your intention?

as title. also improved/fixed some comments

Checklist

• [ ] I have written necessary docs and comments
• [ ] I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #3221 (346fc3b) into main (a4aab7a) will increase coverage by 0.01%.
The diff coverage is 23.65%.

@@            Coverage Diff             @@
##             main    #3221      +/-   ##
==========================================
+ Coverage   73.48%   73.50%   +0.01%     
==========================================
  Files         746      746              
  Lines      101957   101925      -32     
==========================================
- Hits        74923    74915       -8     
+ Misses      27034    27010      -24     

Flag	Coverage Δ
rust	73.50% <23.65%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rontend/src/scheduler/distributed/query_manager.rs	11.32% <ø> (ø)
src/frontend/src/scheduler/local.rs	0.00% <0.00%> (ø)
src/frontend/src/scheduler/mod.rs	0.00% <ø> (ø)
src/frontend/src/scheduler/distributed/stage.rs	49.16% <14.28%> (+1.96%)	⬆️
src/frontend/src/scheduler/distributed/query.rs	69.05% <40.00%> (+1.85%)	⬆️
src/frontend/src/scheduler/plan_fragmenter.rs	93.68% <100.00%> (+0.04%)	⬆️

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[liurenjie1024]

LGTM