Function scan

scan on the list of tensors unpacked from `elems` on dimension 0. The simplest version of `scan` repeatedly applies the callable `fn` to a sequence of elements from first to last. The elements are made of the tensors unpacked from `elems` on dimension 0. The callable fn takes two tensors as

(fn, elems, initializer=None, parallel_iterations=10, back_prop=True,
         swap_memory=False, infer_shape=True, name=None)