Remove the pserver, trainer, evaluators and some useless gradientmachines when compile mobile inference library.

Pass DeviceContext to functors not ExecutionContext.

Implementing the SoftSign activation operator

Add grad_op_desc_builder

Stable elemwise mul

They were using float64 for FP32 kernel before.

Support double precision

* 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

Add reduce op

fix compile error

Use scalar implementation instead of neon implementation to avoid out of range memory access in the tail conv3x3

should reset env every time