This is part of big move of v2 codes to legacy sub dir.

* hierarchical rnn document, add new config example

* update inputs_type of label

* add check for unsupported config

* refine hierarchical document

* refine doc title

* update docs, fix paddle to PaddlePaddle

* follow comments

Sequences should be sorted according to the number of subsequences they have.

Change-Id: I8e0a8ea6fc2760652d9c76440a539c90860062d3