Sequence Scatter Op

In the future, we'd like to encourage user to save
everything during training.
This allows us to
1. Do more flexible optimization passes
2. Re-train and fune-tune

* add unit test for nested drnn

* add nested dyn_rnn

* refine while_op

* fix bug

* Add truncated gaussian initializer.

* Fix unitest.

* Update API.spec

* Fix code style and fix bug.

* Fix code style.

* Small fix.

* Add python api for expand op.

* Fix unitest.

* Remove 'out' from arguments and fix code style.

* fix API.spec

* Fix API

* Fix unitest

* loosen the restriction of output_size in conv2d_transpose

* test and docs

* fix code style

* fix ci test error

* bug fix

* fix python3 issue

* hide program APIs

* fix merge error

* update

* refine seq_softmax_op

* fix seq_softmax

* use cub in seq_softmax

* fix dist table send hang problem

* revert sync_mode config

* fix async send table

* Add sequence_expand_as_op

* follow comment

* Implement slice grad operator. #8130
* test slice grad operator and bug fix
* Fix pre commit style