User can provide an initializer in ParamAttr. The initializer will be called when at parameters.create().

print_layer is used to print the values of a layer when evaluating the model for debugging purpose. No layer depends on print_layer. It should be added to the topology if it is used to print some layer already in the topology.

When using system python, "pip install" needs root previlege. Using virtualenv to avoid this.

change argument name of data_type.integer_value/integer_value_sequence to ele_range, add documentation.