Adds matricize function

[JeanKossaifi]

Adds a function for generalize matricization of tensors. The reciprocate function tensorize still needs to be added.

[codecov]

Codecov Report

Merging #366 (8f03930) into main (eff542a) will increase coverage by 0.19%.
The diff coverage is 95.34%.

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
+ Coverage   88.08%   88.28%   +0.19%     
==========================================
  Files         105      105              
  Lines        6058     6093      +35     
==========================================
+ Hits         5336     5379      +43     
+ Misses        722      714       -8     

Impacted Files	Coverage Δ
tensorly/base.py	95.83% <92.00%> (-4.17%)	⬇️
tensorly/tests/test_core.py	100.00% <100.00%> (ø)
tensorly/tenalg/proximal.py	67.38% <0.00%> (+0.47%)	⬆️
tensorly/decomposition/_cp.py	89.05% <0.00%> (+2.91%)	⬆️

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