Freeze embedding normalization

The embedding normalization kept getting updated. It makes more sense to freeze it.

Related issue: https://github.com/speechbrain/speechbrain/issues/1026

hyperparams.yaml

@@ -34,6 +34,7 @@ classifier: !new:speechbrain.lobes.models.ECAPA_TDNN.Classifier
 mean_var_norm_emb: !new:speechbrain.processing.features.InputNormalization
     norm_type: global
     std_norm: False
+    update_until_epoch: -1  # Freeze the normalization
 
 modules:
     compute_features: !ref <compute_features>