minor bug: dtype in session._apply_spans_src should be based on spans length not target_.dtype

[deng113jie]

dtype in session.apply_spans_src should be based on spans length not target.dtype
For example, the target_.dtype can be int8, if the data in categorical; but when the span is longer than 128, then the result index is going to be out of the number representable by int8.
Hence, the target_.dtype should be depend on the length of span

[deng113jie]

the dest dtype should also depents on the predicate, for example min, max should be the same dtype of src, but index_min or index_max should return the dtype based on src length (span length)

[deng113jie]

More generally, we should have a function return dtype (in utils) for all np.zeros/field construct calls in the code base (mostly in operations.py)