fixes typing errors in auto_restart.py

[donlapark]

What does this PR do?

Fixes typing errors in pytorch_lightning/utilities/auto_restart.py as a part of #13445.

Well, this was quite a tough one, as the type of state_dict is not consistent throughout the file. Sometimes its type is Dict[str, Any], the other times, it is Dict[int, Any].

Also, the state attribute in MergedIteratorState can have two types: Dict[int, IteratorState] or Dict[str, Dict[int, IteratorState]], depending on whether the dataset is map-based or iterable. I have applied a band-aid fix which initially sets the type of state to Dict and then conditionally sets it to a more specific type in update method later.

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[codecov]

Codecov Report

Merging #13904 (e5844dd) into master (66ca093) will increase coverage by 14%.
The diff coverage is 95%.

❗ Current head e5844dd differs from pull request most recent head e81d961. Consider uploading reports for the commit e81d961 to get more accurate results

@@            Coverage Diff            @@
##           master   #13904     +/-   ##
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+ Coverage      61%      76%    +14%     
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  Files         331      331             
  Lines       26621    26709     +88     
=========================================
+ Hits        16343    20253   +3910     
+ Misses      10278     6456   -3822     

[rohitgr7]

@carmocca mind review as code-owner?

[tchaton]

LGTM !