fix updating dict config values

Author: Ghostvv

docs/migration-guide.rst

@@ -92,7 +92,6 @@ General
     - name: DIETClassifier
       hidden_layers_sizes:
         text: [256, 128]
-        label: []
       intent_classification: True
       entity_recognition: False
       use_masked_language_model: False

rasa/nlu/config.py

@@ -59,7 +59,12 @@ def override_defaults(
         cfg = {}
 
     if custom:
-        cfg.update(custom)
+        for key in custom.keys():
+            if isinstance(cfg.get(key), dict):
+                cfg[key].update(custom[key])
+            else:
+                cfg[key] = custom[key]
+
     return cfg