Add free_dimension_overrides in config.json

Based on https://github.com/huggingface/transformers.js/blob/main/src/models.js#L239 , WebNN requires free_dimension_overrides to be set in config.json as a field within "transformers.js_config".

config.json

@@ -40,5 +40,13 @@
   "patch_size": 2,
   "qkv_bias": true,
   "transformers_version": "4.48.2",
+  "transformers.js_config": {
+    "free_dimension_overrides": {
+      "batch_size": 1,
+      "num_channels": 3,
+      "height": 30,
+      "width": 30
+    }
+  },
   "use_mid_position_embeddings": true
 }