Update mnist input type to stablehlo

Update mnist input type to stablehlo to match upstream change.

Change-Id: I3143ff9b608a01cbbce743d8559c6a6aae4092a1

float_models/CMakeLists.txt

diff --git a/float_models/CMakeLists.txt b/float_models/CMakeLists.txt
index 9bf848a..9214957 100644
--- a/float_models/CMakeLists.txt
+++ b/float_models/CMakeLists.txt
@@ -29,7 +29,7 @@
   C_IDENTIFIER
     "float_models_mnist"
   FLAGS
-    "-iree-input-type=mhlo_legacy"
+    "-iree-input-type=stablehlo"
   RVV_OFF
 )