[SYSTEMDS-3979] Split MM Chain into separate Statement Block

[ywcb00]

Hello there,
this PR splits the inputs of a matrix multiplication chain into their own statement block to ensure the MM chain receives correct sparsity information as input (i.e., the sparsity is computed before sparsity rewrites).
The tests are modified accordingly.
All the best,
David

[codecov]

Codecov Report

❌ Patch coverage is 93.05556% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.15%. Comparing base (fc1bf7a) to head (4e2b6ae).

Files with missing lines	Patch %	Lines
.../sysds/hops/rewrite/RewriteSplitDagUnknownNnz.java	92.95%	1 Missing and 4 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##               main    #2623       +/-   ##
=============================================
- Coverage     76.68%   72.15%    -4.54%     
- Complexity        0    51090    +51090     
=============================================
  Files           452     2096     +1644     
  Lines         28066   226485   +198419     
  Branches          0    38581    +38581     
=============================================
+ Hits          21523   163416   +141893     
- Misses         6543    51913    +45370     
- Partials          0    11156    +11156     

Flag	Coverage Δ
java	71.51% <93.05%> (?)
python	76.68% <ø> (ø)

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