“c339946e6db2ad8ba4c2a91edfa2c63d12908135”上不存在“develop/api_doc/v2/fluid.html”
* Fix example code of api `fluid.layers.switch_case` to use api2.0
* add alias `paddle.static.nn.switch_case`
* delete alias `paddle.nn.switch_case` and `paddle.nn.control_flow.switch_case`