Upgrade kf_config.json to v2 (nested config + weight_dtype)

kf_config.json

@@ -1,23 +1,27 @@
 {
   "library_name": "kerasformers",
-  "kerasformers_version": "1.1.3",
+  "kerasformers_version": "1.2.1",
   "model_module": "kerasformers.models.mit",
   "model_class": "MiTImageClassify",
   "variant": "mit_b1_in1k",
   "weights": "model.weights.h5",
+  "schema_version": 2,
+  "weight_dtype": "float32",
   "model_type": "mit",
-  "embed_dim": [
-    64,
-    128,
-    320,
-    512
-  ],
-  "depths": [
-    2,
-    2,
-    2,
-    2
-  ],
-  "image_size": 224,
-  "num_classes": 1000
+  "vision_config": {
+    "embed_dim": [
+      64,
+      128,
+      320,
+      512
+    ],
+    "depths": [
+      2,
+      2,
+      2,
+      2
+    ],
+    "image_size": 224,
+    "num_classes": 1000
+  }
 }