replace frontend/operator/ops.h in backend with base/core_ops.h as
backend should not use any frontend-only primitive

1. Check if data_ is null before compare it;
2. Always allocate an array for data_ even if size is zero.

Refactoring for base and abstract types: Moving Base into base folder; Splitting the abstract type and infer&specialize routines;

Signed-off-by: Nleonwanghui <leon.wanghui@huawei.com>