@Documented @Retention(value=RUNTIME) @Target(value=TYPE) @Experimental(value=SPLITTABLE_DO_FN) public static @interface DoFn.UnboundedPerElement
DoFn
specifying that the DoFn performs an unbounded amount of work per input element, so
applying it to a bounded PCollection will produce an unbounded PCollection. It
is an error to specify this on a non-splittable DoFn.