Add cross-attention cache for dygraph Transformer.
Add greedy search for dygraph Transformer.

* update api to 1.8 for transformer and similarity_net, test=develop

* update readme, test=develop

* Add beam search for dygraph Transformer.
Re-organize dygraph Transformer.

* Add custumed data support for dygraph Transformer.

* Add validation in dygraph Transformer.

* Update notes for multi-gpu for dygraph Transformer.

* using new load api

* merge

* add dygraph user guide in models/dygraph

* remove useless files

* support multi-process for resnet and transformer