Remove OperatorBase::Init

[wangkuiyi]

OperatorBase::Init duplicates with the C++ constructor mechanism. We should remove it.

It seems that people wrote this duplicated mechanism because the constructor of OperatorBase is over-simplified to take no parameters, and OpRegistry::CreateOp would new an operator instance and set OperatorBase:: directly -- the core problem is that OperatorBase's properties are declared public instead of protected.

So, to fix this issue we need to

1. Make OperatorBase's properties protected

so that we need to

1. Add property accessors to OpeatorBase Add OperatorBase property accessors #3426
2. Make OperatorBase and all its sub-classes' constructors able to set property values. Add constructors to OperatorBase and sub-classes #3429
3. Make grad_op_builder.cc reads from OperatorBase:: via the accessors Make grad_op_bulider.cc use OperatorBase::<property-accessor> #3427
4. Make op_registry.{h,cc} call new constructor of OperatorBase. Make op_registry.{h,cc} call new constructor of OperatorBase. #3438
5. Make all writes to OperatorBase:: through constructors.
6. Remove set OperatorBase properties directly.
7. Make properties protected. change operator public to protected #3460

[reyoung]

Done