fix bias concat (#1296)

Co-authored-by: Lin <[email protected]>

src/gluonnlp/model/bert.py

@@ -134,7 +134,11 @@ def hybrid_forward(self, F, qkv, valid_len, query_bias, key_bias, value_bias,
         value_weight = value_weight.reshape(shape=(self._num_heads, -1, 0), reverse=True)
         in_weight = F.concat(query_weight, key_weight, value_weight, dim=-2)
         in_weight = in_weight.reshape(shape=(-1, 0), reverse=True)
-        in_bias = F.concat(query_bias, key_bias, value_bias, dim=0)
+        # concat bias
+        query_bias = query_bias.reshape(shape=(self._num_heads, -1), reverse=True)
+        key_bias = key_bias.reshape(shape=(self._num_heads, -1), reverse=True)
+        value_bias = value_bias.reshape(shape=(self._num_heads, -1), reverse=True)
+        in_bias = F.stack(query_bias, key_bias, value_bias, axis=1).reshape(-1)
 
         # qkv_proj shape = (seq_length, batch_size, num_heads * head_dim * 3)
         qkv_proj = F.FullyConnected(data=qkv, weight=in_weight, bias=in_bias,