[Meta Schedule][BugFix] Fix PostProc RewriteReductionBlock

[MasterJH5574]

This PR fixes a bug in the post-processor RewriteReductionBlock.

Due to my carelessness when implementing the search rule for cross-thread reduction, I mistakenly forgot to return false in the method bool AllReductionIterVarAreUnbound(...). This caused the post-processor never working... Because the method never returns true - the post-processor never extracts the inits from reduction blocks. So sorry for this bug 🙏.

Thanks @spectrometerHBH for reporting this bug ❤️.

@junrushao1994 @zxybazh Now I have two concerns:

1. does the fix conflict the meta-schedule refactor that you guys are doing? If you've already finished the post-processor refactor (which I guess is in [Meta Schedule][M3c] Schedule Rules, Mutator & Postprocs #485), I may need your help to update this fix to the refactored version.
2. I do want to write some unit tests to make sure that the post-processor indeed works before merging this PR. Do you have any suggestions on how to implement such a test?

[junrushao]

LGTM

[junrushao]

1. It’s not conflicted. Don’t worry about it :-)
2. Let’s do that during upstreaming

[zxybazh]

From my point of view, it does not conflict with the current implementation so no worries : )

[MasterJH5574]

Thanks! That's awesome. So let's get it merged first and add tests further during upstream :-)