Deprecate DDPPlugin.task_idx

[carmocca]

What does this PR do?

Legacy attribute

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[codecov]

Codecov Report

Merging #8203 (3067860) into master (47c7654) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #8203   +/-   ##
======================================
- Coverage      93%     92%   -0%     
======================================
  Files         212     212           
  Lines       13588   13595    +7     
======================================
- Hits        12626   12566   -60     
- Misses        962    1029   +67