[Cherry-Pick][CI] Remove splitwise_scheduler and download latest_wheel explicitly to avoid pip cache issues(#6250)

[EmmonsCurse]

Motivation

When installing PaddlePaddle from a direct URL, pip may reuse cached wheels and fail to pick up the latest self-built package, leading to incorrect test coverage in CI.

Modifications

• Download the wheel package explicitly before installation
• Remove unit test of splitwise_scheduler

Usage or Command

N/A

Accuracy Tests

N/A

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (release/2.4@4097455). Learn more about missing BASE report.

Additional details and impacted files

@@              Coverage Diff               @@
##             release/2.4    #6265   +/-   ##
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  Coverage               ?   57.60%           
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  Files                  ?      330           
  Lines                  ?    41302           
  Branches               ?     6289           
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  Hits                   ?    23790           
  Misses                 ?    15657           
  Partials               ?     1855           

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