[core] const char* should be removed in favor of string

In Tensor, setBackend is expecting an std::string which is passed to the create() function. In TensorImpl, the Backend is passed as a mere pointer to char which might go dangling if the corresponding used string is not permanent.

if std::string is allowed, shouldn't it be used everywhere to guarantee correct memory management? Otherwise, char const * based signatures will probably lead to runtime errors.

TODO:

• Operators name
• Operators type
• Operator attributes
• Data mType