[SD] Add flag to collapse reduction dims pre dispatch formation (#1999)

apps/stable_diffusion/src/utils/utils.py

@@ -563,6 +563,9 @@ def get_opt_flags(model, precision="fp16"):
         iree_flags += opt_flags[model][is_tuned][precision][
             "specified_compilation_flags"
         ][device]
+    # Due to lack of support for multi-reduce, we always collapse reduction
+    # dims before dispatch formation right now.
+    iree_flags += ["--iree-flow-collapse-reduction-dims"]
     return iree_flags