modify the format

2. Add crop layer unitest
3. Fix bugs

* Fix  #2797
* It because trainer_config_helpers' __dfs_travel__ did not record the
  node which travelled, and if the topology has a recursive dependency,
  there are some nodes will be travelled multiple times.
* Add a `travelled` set to record which node is travelled.
* Also add a unittest for this situation.

Fix issue #2562

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