Drop failure case for stablehlo.dynamic_broadcast_in_dim

[rsuderman]

The failure to broadcast dynamically makes the assumption the input dynamic shape could be expanded by being 1. This should be handled by an earlier trasform to materialize a known broadcast if we intend to support both cases.

[rsuderman]

@GleasonK if you do not mind looking over. I believe there is a way to safely introduce the conditional broadcasting but it depends on how stablehlo.get_dimension_size is lowered.

[jpienaar]

Could you say more about requirement on lowering of get_dimension_size? Also is that what's happening at the moment via the lowering to linalg path?

[rsuderman]

Could you say more about requirement on lowering of get_dimension_size? Also is that what's happening at the moment via the lowering to linalg path?

There is no current lowering in the stablehlo repo. iree side we just lower to tensor.dim and tensor.from_elements instead of concatting everything together.

Something worth noting is to have the conditional broadcast (e.g. ? being a broadcasting 1) we could materialize out an additional 1 dim then broadcast across it.