Fix models for sequences of infinite element type (#6870)

Fix models for sequences of infinite element type (#6870)

This fixes our model construction for sequences of infinite element type.

We were relying on getModelValue in our model construction which is incorrect since it assumes that the element type's theory can provide concrete values during model generation time. This makes the sequence model construction more robust by generalizing how model values are assigned: we use skeletons instead of concrete values when the element type is infinite.

This fixes some open model generation issues with Facebook benchmarks.