[Algorithm] Add TwoTower model

[massquantity]

No description provided.

[codecov]

Codecov Report

Merging #285 (5ec5717) into master (b327c85) will decrease coverage by 0.02%.
The diff coverage is 98.61%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
- Coverage   99.42%   99.40%   -0.02%     
==========================================
  Files         111      112       +1     
  Lines        6903     7040     +137     
==========================================
+ Hits         6863     6998     +135     
- Misses         40       42       +2     

Flag	Coverage Δ
CI	99.40% <98.61%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
libreco/algorithms/two_tower.py	98.13% <98.13%> (ø)
libreco/algorithms/__init__.py	100.00% <100.00%> (ø)
libreco/algorithms/rnn4rec.py	100.00% <100.00%> (ø)
libreco/batch/batch_data.py	100.00% <100.00%> (ø)
libreco/batch/tf_feed_dicts.py	100.00% <100.00%> (ø)
libreco/tfops/loss.py	100.00% <100.00%> (ø)
libreco/training/trainer.py	100.00% <100.00%> (ø)
libreco/utils/constants.py	100.00% <100.00%> (ø)
libreco/utils/validate.py	100.00% <100.00%> (ø)

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