Merge pull request #6790 from jacquesqiao/kernel-hint-design

Kernel hint design

doc/design/kernel_hint_design.md

+## Problem
+In PaddlePaddle's [Design](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md), one Operator may have multiple kernels. Users may have some personal preference to choose a certain type of kernel for an operator, such as `force_cpu` to choose a CPU kernel, `use_cudnn` to choose a CUDNN kernel, we need to provide a way for users to do this.
+
+In the current design, we use KernelType to describe one kernel.
+
+```cpp
+struct KernelType {
+  Place place_;
+  DataType data_type_;
+  LayoutType layout_;
+};
+```
+ `place_` `data_type_` and `layout_` can be got from the input tensors of the operator, `GetActualKernelType(inputs)` use inputs to infer the proper kernel key that fit the incoming data, but users can not directly configure it.
+
+The [design](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md) also provides a virtual method `GetExpectedKernelType` that user can overload and use to choose the KernelType they want to use.
+
+So we should send the information user defined in proto to `GetExpectedKernelType` for choosing a kernel.
+
+The problem is, how should we define and send the information for `GetExpectedKernelType` to use?
+
+## Solution
+
+### Potential choice
+1. Do nothing, let the user add the information they want to operator‘s attribute and get them inside `GetExpectedKernelType`, this can work properly. But there is a little problem that users may define many kinds of hints for the same purpose, such as `force_cpu`, `use_cpu`, `cpu_kernel` to choose CPU kernel, and `use_cudnn`, `force_cudnn`, `cudnn_kernel` to choose CUDNN kernel.
+
+2. Pre-define all the needed option and use a single attr key such as `kernel_hint` for the user, this is not so flexible if the user wants to define some more kind of hint.
+
+### Final choice
+To provide enough flexibility while avoiding confusion definition, we can define some global constants for these attribute names, such as `force_cpu`, `use_cudnn`, `use_mkldnn` for a user to choose.
+
+In C++
+
+```cpp
+const std::string kForceCPU = "force_cpu";
+const std::string kUseCUDNN = "use_cudnn";
+const std::string kUseMKLDNN = "use_mkldnn";
+
+KernelType GetExpectedKernelType() {
+  if (Attr<bool>(kForceCPU)) {
+    return KernelType(CPUPlace, ...)
+  } else {
+    ...
+  }
+}
+```
+
+In Python code
+
+```python
+FORCE_CPU = core.kForceCPU()
+
+def xx_layer(..., force_cpu=false):
+  layer_helper = LayerHelper(...)
+  layer_helper.append_op(
+    type="xx",
+    attr={FORCE_CPU: force_cpu})
+```