Support TFNet Training (#451)
Support TFNet Training (#451)

* support tfnet training

* fix memory leak

* remove useless code

* test

* fix bugs

* add test

* add back

* refine test name

* fix sparse gradient

* serialization

* support saving model

* remove test

* add comments

* fix tests

* fix test

* fix test

* fix tests

* address comments

* address comments

* refactor

* refactor

* remove serilization test

* fix python

* meet code review

* fix teset

* fix test

* fix test

* fix style

* fix test

* fix style

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