test=develop

* update api_cn; fix the error of reverting layers_cn to 1.4; include previous changes

* update layers_cn; add code example to elementwise_mod

* delete recordio_writer_cn; revert index_cn

* update api_cn after review

* fix io_cn.rst

* update layers_cn

* update io_cn

* fix textual content

* fix issue #682 3 3.2

* fix issues in conv