* Add normalization operator.
1. Refine the raw norm_op and let it more general to support to normalize Tensor along any axis.
2. There is a bug in l2_normalize API, which lacks sqrt after `reduce_sum`.
3. Use norm_op to refine the l2_normalize API.
4. Fix bug in test_normalization_wrapper.py.

* Add slice op.

* Remove using from header file and fix doc.

* Fix doc

* Small fix.

* Expose maxout API.

* Fix code style.

* Auto generate python api for polygon_box_transform op.

* Add unitest.

* fix conflict

* code clean

* Add python wrapper for gather op.

* Add unitest for 'rank==1' and fix comments.

* Fix comments.

* Add shape op to get the shape of variable.

* Rename get_shape to shape.

* Add checker for output and fix comments.

* Make bilinear_interp_op support attrs from input.

* Fix python api.

* unify UpsamplingBilinear2d interface specification

* unify UpsamplingBilinear2d interface specification

* fix name conventions

* small fix about computation order