Add per-stage predictor config for FLUX stage2 (required by from_pretrained)

flux/best_predictor_stage2_config.json

@@ -0,0 +1,11 @@
+{
+  "dim": 3072,
+  "num_blocks": 1,
+  "hidden_dim": 128,
+  "split_dims": [
+    2304,
+    384,
+    384
+  ],
+  "dropout": 0.0
+}