Remove TF imports  (#2487)
Remove TF imports  (#2487)

* remove imports

* add check code

* refine

* check at import

* remove useless import

* fix style

* fix bug

* fix tests

* fix tests

* revert ZooOptimizer
intel-analytics/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL