* Optimizer trans momentum (#29597)

* merge amp related function in Momentum from paddle.fluid.contrib.optimizer into paddle.optimizer.

* Add unittest for 2.0  Momentum API.

* fix some bugs in weight_decay.

* add alias for fluid.contrib.mixed_precision (#29562)

* add alias for fluid.contrib.mixed_precision

* add static.amp into setup.pu.in (#29621)

* add static.amp into setup.pu.in

* add unittest for api

* fix a bug in multi_precision_fp16 unittest. (#29756)

* add onnx export module, test=develop

* add unit test for paddle.onnx.export

* adjust api & doc

* fix some typo

* test=develop, bug fix

* test=develop, util documents

* fix api document

* move paddle.fleet to paddle.distributed.fleet

* test=develop,test=document_fix, remove the out args

* fleet_util move to paddle.fleet
Co-authored-by: NWuHaobo <wuhaobo1994@gmail.com>
Co-authored-by: Ntangwei12 <tangwei12@baidu.com>

refactor fleet api under paddle.fleet
update DistributedStrategy

* add paddle.fleet.DistributedStrategy for 2.0

do some odd jobs, test=develop

* add launch mp distributed mode module test=develop

* delete unused file test=develop

* refine usage test=develop

* refine usage test=develop

* move distributed package test=develop

* add to whl package test=develop

* Init slim.

* Remove distillation demo.

* Fix import errors.
test=develop

* Fix some issues.
test=develop

* Fix configs.
test=develop

* Modify API.spec.
test=develop

* Fix format.
test=develop

* Fix format.
test=develop

* Add some comments.

ISSUE=4586495

git-svn-id: https://svn.baidu.com/idl/trunk/paddle@1408 1ad973e4-5ce8-4261-8a94-b56d1f490c56