Update data_type.py (#238)

onnx_tf/common/data_type.py

@@ -10,7 +10,16 @@ def tf2onnx(dtype):
   if isinstance(dtype, Number):
     tf_dype = tf.as_dtype(dtype)
   elif isinstance(dtype, tf.DType):
-    tf_dype = dtype
+    # Usually, tf2onnx is done via tf_type->numpy_type->onnx_type
+    # to leverage existing type conversion infrastructure;
+    # However, we need to intercept the string type early because 
+    # lowering tf.string type to numpy dtype results in loss of 
+    # information. <class 'object'> is returned instead of the 
+    # numpy string type desired.
+    if tf_dype is tf.string:
+      return TensorProto.STRING
+    else:
+      tf_dype = dtype
   else:
     raise RuntimeError("dtype should be number or tf.DType.")
   return mapping.NP_TYPE_TO_TENSOR_TYPE[np.dtype(tf_dype.as_numpy_dtype)]