Use global_step while restoring logging step for old checkpoints

[rohitgr7]

What does this PR do?

Here #13467, we fixed the issue by restoring the logging step from the checkpoint, but for the cases where old checkpoints (<v1.6.5) are used, the behavior will still be the same because that key will not be present in old checkpoints and logging will start from 0. To avoid this, the default behavior is changed to restore global_step instead.

Discussed with @carmocca

Does your PR introduce any breaking changes? If yes, please list them.

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cc @Borda @carmocca @edward-io @ananthsub @rohitgr7 @kamil-kaczmarek @Raalsky @Blaizzy

[tchaton]

Nice catch !

[akihironitta]

LGTM!

[codecov]

Codecov Report

Merging #13645 (0c73b63) into master (2845e75) will decrease coverage by 10%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13645     +/-   ##
=========================================
- Coverage      86%      76%    -10%     
=========================================
  Files         326      327      +1     
  Lines       25394    25512    +118     
=========================================
- Hits        21839    19446   -2393     
- Misses       3555     6066   +2511