Need a new class to represent some certain interface of Table used in lookup remote table

[Yancey1989]

For the feature lookup remote table, we use the variable type SelectedRows to represent a Table currently. But sometimes the code is not certainly enough. Maybe we need a new class named Table(or othes) that inherit SelectedRows and implement some certain interface, the pseudo code as following:

class Table : public framework::SelectedRows {
bool HasKey(int64_t key)
bool Resize(framework::ddim shape); // resize value and rows size
Tensor Get(int64_t key)
bool Set(int64_t key, Tensor value);
...
}

[Yancey1989]

As the offline discussion, maybe we can SelectedRows always, and implement some new interfaces to operate the sparse table. And on the other hand, we can auto-reallocate the memory while calling the Set interface.