[Unified Checkpoint] bug fix

[DesmonDay]

PR types

Bug fixes

PR changes

Others

Description

多机环境下由于代码改动出现问题，optimizer.safetensors.index.json可能不存在，修复。

[paddle-bot]

Thanks for your contribution!

[ZHUI]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 53.02%. Comparing base (297dbce) to head (fad0f64).
Report is 267 commits behind head on develop.

Files with missing lines	Patch %	Lines
...p/trainer/unified_checkpoint/unified_checkpoint.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #9669      +/-   ##
===========================================
+ Coverage    52.79%   53.02%   +0.22%     
===========================================
  Files          718      718              
  Lines       112241   112243       +2     
===========================================
+ Hits         59262    59521     +259     
+ Misses       52979    52722     -257     

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