1. v2.layer.parse_network does not correctly handle the generation output.
2. GatherAgentLayer does not correctly handle generation output when batch_size > 1
3. Fix CustomStackTrace for rnn group

For example:
layer.printer(input=[x, y], format="x=%s y=%s")

Also remove active_type argument for many layers in config_parser.py because it is automatically
handled by LayerBase.

No longer need to specify target_inlinks or is_seq.

No longer need to use SubsequenceInput. The framework will detect.

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

images reader: read the data without untarring the tarball file.
image.py: move batch function from reader to image.py

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.