Change Dataset interface to support sparse matrix.

[eugene-yang]

Changed the libact.base.dataset.Dataset interface to support sparse matrix as X. The interfaces for get_entries, get_labeled_entries and get_unlabled_entries are changed. Since most of the usage of these methods are getting the list of tuple and zip them back to the a feature matrix and list of labels, directly change the interface to output in this format would benefit both using and storing the data.

This would also directly support scipy.sparse.csr_matrix since the zipping during the initialization is removed. The interface of Dataset.data[] is still implemented via __getitem__ magic method to support some of the use case that involve direct access to the entries.

[codecov-io]

Codecov Report

Merging #165 into master will decrease coverage by <.01%. The diff coverage is 98.52%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
- Coverage   89.46%   89.46%   -0.01%     
==========================================
  Files          37       37              
  Lines        1557     1566       +9     
==========================================
+ Hits         1393     1401       +8     
- Misses        164      165       +1

Impacted Files	Coverage Δ
...ery_strategies/multiclass/hierarchical_sampling.py	95.85% <100%> (ø)	:arrow_up:
...query_strategies/multilabel/binary_minimization.py	100% <100%> (ø)	:arrow_up:
...ct/query_strategies/active_learning_by_learning.py	85.71% <100%> (ø)	:arrow_up:
...ltilabel/cost_sensitive_reference_pair_encoding.py	92.1% <100%> (ø)	:arrow_up:
libact/query_strategies/variance_reduction.py	68.88% <100%> (-1.33%)	:arrow_down:
libact/labelers/ideal_labeler.py	100% <100%> (ø)	:arrow_up:
libact/query_strategies/hintsvm.py	93.02% <100%> (ø)	:arrow_up:
...es/multilabel/multilabel_with_auxiliary_learner.py	89.58% <100%> (ø)	:arrow_up:
libact/models/multilabel/dummy_clf.py	94.11% <100%> (ø)	:arrow_up:
.../multiclass/active_learning_with_cost_embedding.py	100% <100%> (ø)	:arrow_up:
... and 10 more

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[yangarbiter]

The changes looks good to my. Thanks for the contribution. Just fix the coding style (mainly about whitespaces) and I think it's be ready to merge.

[eugene-yang]

@yangarbiter Do you want me to fix them? Like the whitespaces around the brackets?

[yangarbiter]

Yes, please fix them For example lb = lbr.label( trn_ds.data[ask_id][0] ) should be lb = lbr.label(trn_ds.data[ask_id][0]) I didn't mark all of them. But try to comply with google's style guide (https://google.github.io/styleguide/pyguide.html#36-whitespace) Thanks.

[yangarbiter]

Last two questions and I'll merge. Thanks for the hard work @eugene-yang .