add 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 use a CPU kernel, `use_cudnn` to choose a CUDNN kernel, we need to provide a way for a user 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 come from the input tensor of the operator, `GetActualKernelType(inputs)` use inputs to infer the proper kernel key that fit the incoming data, user can not config it.
+
+The design also provides a virtual method `GetExpectedKernelType` that user can overload and 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
+1, Do nothing, let the user add the information they want to operator‘s attribute and get them inside `GetExpectedKernelType`, this can work right. 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` for CPU kernel, and `use_cudnn`, `force_cudnn`, `cudnn_kernel` for use of 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.
+
+
+To provide enough flexibility while avoiding confusion definition, we can predefine some options, such as `force_cpu`, `use_cudnn`, `use_mkldnn` for a user to choose.
+
+```cpp
+const std::string kNonHint = "";
+const std::string kForceCPU = "force_cpu";
+const std::string kUseCUDNN = "use_cudnn";
+const std::string kUseMKLDNN = "use_mkldnn";
+
+KernelType GetExpectedKernelTyp() {
+    // "kernel_hint" is a user defined attribute name
+	if (Attr<std::string>("kernel_hint") == kForceCPU) {
+		return KernelType(CPUPlace, ...)
+	} else {
+		...
+	}
+}
+```
+
+In Python code
+
+```python
+def xx_layer(..., kernel_hint=None):
+	layer_helper = ...
+	layer_helper .append_op(
+		type="xx",
+		# "kernel_hint" should be the same with the attr name in CPP
+		attr={"kernel_hint": kernel_hint or ""})
+```