Missing matrix imputation with TruncatedSVD; add and edit test cases

[cyrillustan]

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[codecov]

Codecov Report

Merging #27 (5a0f27c) into main (b351770) will increase coverage by 3.22%.
The diff coverage is 98.03%.

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   76.64%   79.87%   +3.22%     
==========================================
  Files          10        9       -1     
  Lines         578      616      +38     
==========================================
+ Hits          443      492      +49     
+ Misses        135      124      -11     

Flag	Coverage Δ
unittests	79.87% <98.03%> (+3.22%)	⬆️

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Impacted Files	Coverage Δ
tensorpack/decomposition.py	91.80% <95.65%> (+2.32%)	⬆️
tensorpack/test/test_decomposition.py	100.00% <100.00%> (ø)

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