Fix MatrixFactorization trainer's warning

[LittleLittleCloud]

Problem to fix

MatrixFactorization's default options will throw a warning msg Warning: insufficient blocks may slow down the trainingprocess (4*nr_threads^2+1 blocks is suggested) when running on PC with more than 10 threads.

Cause

The warning msg is from libmf, which will check nr_bin and nr_thread, if nr_bin <= 2* nr_thread, it will show that warning.

MatrixFactorization uses max(20, 2*nr_thread) to initialize nr_bin, when nr_thread < 10, it's fine because nr_bin will be 20, which is greater than 2*nr_thread, but if nr_thread is >= 10, nr_bin's value will be 2*nr_thread, which is less than 2*nr_thread + 1 and trigger warning.

Fix

initialize nr_bin with 2 * nr_threads + 1.

Related issue

#5067

[codecov]

Codecov Report

Merging #5071 into master will decrease coverage by 0.18%.
The diff coverage is 54.95%.

@@            Coverage Diff             @@
##           master    #5071      +/-   ##
==========================================
- Coverage   75.83%   75.65%   -0.19%     
==========================================
  Files         951      993      +42     
  Lines      172613   178157    +5544     
  Branches    18632    19124     +492     
==========================================
+ Hits       130904   134786    +3882     
- Misses      36529    38145    +1616     
- Partials     5180     5226      +46     

Flag	Coverage Δ
#Debug	75.65% <54.95%> (-0.19%)	⬇️
#production	71.62% <54.95%> (+0.17%)	⬆️
#test	88.67% <ø> (-1.83%)	⬇️

Impacted Files	Coverage Δ
....AutoML/API/RunDetails/CrossValidationRunDetail.cs	100.00% <ø> (ø)
...rc/Microsoft.ML.AutoML/API/RunDetails/RunDetail.cs	95.45% <ø> (ø)
....AutoML/ColumnInference/ColumnGroupingInference.cs	85.00% <ø> (ø)
...t.ML.AutoML/ColumnInference/ColumnTypeInference.cs	85.46% <ø> (+1.76%)	⬆️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	62.25% <0.00%> (ø)
...icrosoft.ML.AutoML/Experiment/SuggestedPipeline.cs	88.65% <ø> (ø)
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	83.19% <ø> (ø)
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	84.32% <ø> (ø)
src/Microsoft.ML.AutoML/Sweepers/SmacSweeper.cs	93.47% <ø> (ø)
src/Microsoft.ML.AutoML/Utils/BestResultUtil.cs	57.53% <0.00%> (-1.63%)	⬇️
... and 363 more