Shrink over-allocated sparse layers to final size

[drhagen]

This is just good hygiene and ensures that output tensors do not take up more memory than they need to. The realloc probably causes a slight slowdown, but that is untested.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.78%. Comparing base (a082486) to head (34778da).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
+ Coverage   90.74%   90.78%   +0.04%     
==========================================
  Files          60       60              
  Lines        3165     3179      +14     
  Branches      386      389       +3     
==========================================
+ Hits         2872     2886      +14     
  Misses        209      209              
  Partials       84       84              

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