* Make cast op support bool

Also add `elemwise_sub/mul/abs/clip` layers

* Make fuild.layers as a module

* Move layers as a module

* Split layers.py into layers module

* Fix CI

* Fix CI

update the link of docs icon

Unify `step_block` and `block` to `sub_block`

* "derived cudnnDevice context"

* "leave remove cudnn handle from CUDADeviceContext"

* "fix math function error"

Enhance Parameters.init_from_tar in V2

Enhance ReduceOp to support reducing over all elements

Refine conv

update paddledev to paddlepaddle

* Add fluid.md

* in response to comments from Tony and Abhinav

* add support new device doc

* add more info

Christmas gift: fix a typo

remove ATLAS library

Also add `elemwise_sub/mul/abs/clip` layers

Fix #6531

typo WITH_TEST to WITH_TESTING

* "add registry interface"

* "move function to registry"

* "rename with meaningful name"

* "add exposed layers"

* "fixed based on comments"

* "remove unsed comments"

* first commit

* Update read_source.md

* fix the ending symbol

* fix invalid content link

add prelu neon impl

fix warning in row_conv_op.cu

* Add fill_op

* Fix bug

merge Release/0.11.0 to develop

* update docs about how to add a new operator

There are mainly following fixes:

- take `DeviceContext` as the template parameter of math functors and OpKernel instead of `Place`
- remove `eigen_device` interface in base class  `DeviceContext`
- remove `GetEigenDevice` interface in `ExecutionContext` and base class `DeviceContext`
- remove unused `platform::EigenDeviceConverter`
- rename `REGISTER_OP_GPU_KERNEL` to `REGISTER_OP_CUDA_KERNEL`
- rename `USE_GPU_ONLY_OP` to `USE_CUDA_ONLY_OP`