Fix the checkpoint name to load for DDP.

[workingloong]

What changes were proposed in this pull request?

Fix to checkpoint name to load for DDP.

Why are the changes needed?

All ranks of DDP should load the checkpoint of "rank_0.pt".

Does this PR introduce any user-facing change?

No.

How was this patch tested?

UT.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 78.53%. Comparing base (38fee4f) to head (8df90a5).

Files	Patch %	Lines
...rover/trainer/torch/flash_checkpoint/ddp_engine.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1034   +/-   ##
=======================================
  Coverage   78.52%   78.53%           
=======================================
  Files         187      187           
  Lines       17329    17336    +7     
=======================================
+ Hits        13608    13615    +7     
  Misses       3721     3721           

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