config: prepare for upcoming umT5 integration

config.json

@@ -15,7 +15,7 @@
   "is_encoder_decoder": true,
   "is_gated_act": true,
   "layer_norm_epsilon": 1e-06,
-  "model_type": "t5",
+  "model_type": "umt5",
   "num_decoder_layers": 8,
   "num_heads": 6,
   "num_layers": 8,