[BUGFIX] Avoid using dict for attention cell parameter creation (#1050)

Workaround for apache/mxnet#17056

src/gluonnlp/model/attention_cell.py

@@ -217,8 +217,8 @@ def __init__(self, base_cell, query_units, key_units, value_units, num_heads, us
         self._base_cell = base_cell
         self._num_heads = num_heads
         self._use_bias = use_bias
-        units = {'query': query_units, 'key': key_units, 'value': value_units}
-        for name, unit in units.items():
+        units = [('query', query_units), ('key', key_units), ('value', value_units)]
+        for name, unit in units:
             if unit % self._num_heads != 0:
                 raise ValueError(
                     'In MultiHeadAttetion, the {name}_units should be divided exactly'