Add ddp_notebook alias for ddp_fork

[awaelchli]

What does this PR do?

Follow up to #13405

After introducing the DDP Fork method, we can add a user-friendly alias "ddp_notebook" for the primary use case in Jupyter notebooks. This was requested internally (for those who have access).
It is just an alias in the registry. There will be a different PR that adds auto-detection in Jupyter notebooks.

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cc @Borda @justusschock @kaushikb11 @awaelchli @akihironitta @rohitgr7

[codecov]

Codecov Report

Merging #13744 (177c19b) into master (c3299d2) will decrease coverage by 10%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #13744     +/-   ##
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- Coverage      86%      76%    -10%     
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  Files         327      327             
  Lines       25560    25562      +2     
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- Hits        21969    19523   -2446     
- Misses       3591     6039   +2448