fix head_mask for albert encoder part(`AlbertTransformer`) (#11596)

* fix head mask for albert encoder part

* fix head_mask for albert encoder part

src/transformers/models/albert/modeling_albert.py

@@ -450,6 +450,8 @@ class AlbertTransformer(nn.Module):
         all_hidden_states = (hidden_states,) if output_hidden_states else None
         all_attentions = () if output_attentions else None
 
+        head_mask = [None] * self.config.num_hidden_layers if head_mask is None else head_mask
+
         for i in range(self.config.num_hidden_layers):
             # Number of layers in a hidden group
             layers_per_group = int(self.config.num_hidden_layers / self.config.num_hidden_groups)