Following the design
* https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/python_api.md

Just written `Program`, `Block` and unittest of program.

* Changing learning rate from attribute to input(float)
* Removing obsolete code

* init remove cond_op infershape

* optimize code

* add PrepareDataForSubnet and MergeDataFromSubnet

They were using float64 for FP32 kernel before.

They were using float64 for FP32 kernel before.

gather scatter gpu

* Adding support for the sigmoid_cross_entropy_with_logits operator

* Fixing a typo in the cuda file

* Adding Python documentation for sigmoid_cross_entropy_with_logits_op

* Correcting typos in documentation

* Adding unit tests for sigmoid_cross_entropy_with_logits_op

* Addressing code review feedback

User StridedMemCpy in Concat/Split Op

InferShape in Operator should be performed in OperatorBase::Run.

* cond_op, recurrent_op and mnist might be changed in following PR