Fix infer when input is empty in v2/inference.py

refine "Testing Backward Operators" in new_op_cn/en.md

fix typo

remove rowwise_add_op

fix random seeds in gradient checkers.

Add sequence softmax operator.

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