SparseLinear SparseJoinTable DenseToSparse (#1652)
SparseLinear SparseJoinTable DenseToSparse (#1652)

* SparseLinear SparseJoinTable DenseToSparse

* Python api

* add DenseToSparseSpec

* update to upstream

* add some method

* meet code review

* fix python unit test

* fix python unit test

intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL