enable operator gpu unittest

Fix pre-commit

cpu/gpu mean op and its unit test

add check in OperatorContext Input/Output

Add Backtrace for enforce

Feature/backward

* add start_record interface

* call master client in reader

* update

* add demo code in comments

* update comments

* delete unittest for recordio reader

Slice Layer

ENH: Add EigenScalar

Refactorize Scope

* Make interface of Operator to `const Scope&`

* use operator context

* optimize code

* update net infershape

* update InferShape

* disable override InferShape(scope) in OperatorBase

* change InferShapeImpl to InferShape

* add template to OperatorContext Input/Output

* merge Input InputVar, Output OutputVar

* change Inputs to MultiInput

* fix conflict

* fix MultiInput bugs and add unit test

* rename KernelContext to ExecutionContext

* clean code

* change InferShape to protected

* fix template bug

* refine code

* use InputVar instead of Input<Variable>

* typo

* optimize code