Created by: JiayiFeng

In our current design, a C++ reader has an interface named HasNext(), which is used to indicate whether the next batch data exists.

However, this design has at least two issues:

1. There is no method to force users to invoke HasNext() in advance before invoking ReadNext(). We still need some mechanisms in ReadNext() to deal with the EOF status.

2. We are using a CSP channel as readers' double buffers. But CSP channel has no interface to check whether it is empty, so HasNext() is hard to implement on readers with a double buffer.

So I think the HasNext() is redundant. We can remove it and return an empty vector<LoDTensor> in ReadNext() to indicate it's the end of a file.