Porting Sub Sequence Operator

[wanghaox]

The SubSequence operator crop a subsequence from given sequence with given start offset and subsequence size.
It only supports sequence (LoD Tensor with the level number is 1).

Case:

    LoD(x) = {{0, 3, 6, 10}}; Dims(x0) = (10, 3, 2)
    offset = (0, 1, 1); size = (2, 1, 2)
    LoD(Out) = {{0, 2, 3, 5}}; Dims(Out) = (5,3,2)