Refactor some logic for prepare

[ouyuanning]

What type of PR is this?

• [ ] API-change
• [ ] BUG
• [ ] Improvement
• [ ] Documentation
• [ ] Feature
• [ ] Test and CI
• [x] Code Refactoring

Which issue(s) this PR fixes:

issue #14940

What this PR does / why we need it:

Refactor some logic for prepare 1、clean pipeline的时候，如果是prepare的话，使用reset，而不是free 2、把get readers抽取出来，方便后面多次execute的时候可以继续使用