Fix SkipLayerNorm for 2D input (#17014)
Fix an obvious bug:
(1) In packing mode, the input for SLN has two dimensions (introduced by
#15283): [token_count, hidden_size]. Current code of `element_count =
input_dims[0] * sequence_length * hidden_size` will use element_size =
token_count * hidden_size * hidden_size, and causes invalid memory write
in cuda kernel and ORT crash
and two minor issues:
(2) potential integer overflow in `static_cast<int>(element_count)`
(3) some dead code after `return LaunchSkipLayerNormKernel` that will
never have chance to run.
