This op enables expressing uncertainty regarding what should be
happening at particular places in transform-dialect schedules. In
particular, it enables representing a choice among alternative regions.
This choice is resolved through providing a `selected_region` argument.
When this argument is provided, the semantics are such that it is valid
to rewrite the op through substituting in the selected region -- with
the op's interpreted semantics corresponding to exactly this.
This op represents another piece of the puzzle w.r.t. a toolkit for
expressing autotuning problems with the transform dialect. Note that
this goes beyond tuning knobs _on_ transforms, going further by making
it tunable which (sequences of) transforms are to be applied.
A new transform op to represent that an attribute is to be chosen from a
set of alternatives and that this choice is made available as a
`!transform.param`. When a `selected` argument is provided, the op's
`apply()` semantics is that of just making this selected attribute
available as the result. When `selected` is not provided, `apply()`
complains that nothing has resolved the non-determinism that the op is
representing.
