Fix doc

python/mxnet/contrib/quantization.py

@@ -452,9 +452,11 @@ def quantize_model(sym, arg_params, aux_params,
         otherwise, no information of the layer's output will be collected. If not provided,
         all the layers' outputs that need requantization will be collected.
     logger : Object
+        A logging object for printing information during the process of quantization.
 
     Returns
-    `(qsym, qarg_params, aux_params)`
+    -------
+    tuple
         A tuple of quantized symbol, quantized arg_params, and aux_params.
     -------
     """
@@ -471,7 +473,8 @@ def quantize_model(sym, arg_params, aux_params,
             idx = nodes.list_outputs().index(sym_name + '_output')
             excluded_syms.append(nodes[idx])
     logger.info('Quantizing symbol')
-    qsym = _quantize_symbol(sym, excluded_symbols=excluded_syms, offline_params=arg_params.keys())
+    qsym = _quantize_symbol(sym, excluded_symbols=excluded_syms,
+                            offline_params=list(arg_params.keys()))
 
     logger.info('Quantizing parameters')
     qarg_params = _quantize_params(qsym, arg_params)