[MLIR][linalg] Fix unpack rewriter for dynamic shapes (#67096)

Prior to this patch, `GeneralizeOuterUnitDimsUnPackOpPattern` would
assert that we cannot create a `tensor.empty` operation with dynamic
shapes.

The problem stems from the fact that we were not using the right builder
for the `tensor.empty` operation. Indeed, each dynamic dim needs to be
specified by an input variable.

Simply provide the dynamic dimensions to the `tensor.empty` builder to
fix that.