add new_op_kernel_en doc (#7681)

* add new_op_kernel_en.md

doc/howto/dev/new_op_kernel_en.md

+## Add Kernels for a New Device
+
+### Background
+
+PaddlePaddle Fluid have hundreds of operators.  Each operator could have one or more kernels.  A kernel is an implementation of the operator for a certain device, which could be a hardware device, e.g., the CUDA GPU, or a library that utilizes a device, e.g., Intel MKL that makes full use of the Xeon CPU.
+
+[This document](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/howto/dev/new_op_en.md) explains how to add an operator, and its kernels.  The kernels of an operator are indexed by a C++ type [`OpKernelType`](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/operator_kernel_type.md).  An operator chooses the right kernel at runtime.  This choosing mechanism is described [here](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md).
+
+### Write Kernels for A New Device 
+
+#### Add A New Device
+
+  For some historical reaons, we misuse the word *library* for *device*.  For example, we call the deivce type by *library type*.  An example is the header file [`library_type.h`](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/library_type.h#L24).  We will correct this ASAP.
+
+To register a new device, we need to add an enum value to `LibraryType`:
+
+```
+enum class LibraryType {
+  kPlain = 0,
+  kMKLDNN = 1,
+  kCUDNN = 2,
+};
+```
+
+
+#### Add A New [Place](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/platform/place.h#L53)
+
+If you have a new kind of Device, firstly you need to add a new kind of [`Place`](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/platform/place.h#L53). For example `CUDAPlace`:
+
+```cpp
+struct CUDAPlace {
+  CUDAPlace() : CUDAPlace(0) {}
+  explicit CUDAPlace(int d) : device(d) {}
+
+  inline int GetDeviceId() const { return device; }
+  // needed for variant equality comparison
+  inline bool operator==(const CUDAPlace &o) const {
+    return device == o.device;
+  }
+  inline bool operator!=(const CUDAPlace &o) const { return !(*this == o); }
+
+  int device;
+};
+
+typedef boost::variant<CUDAPlace, CPUPlace> Place;
+```
+
+#### Add [device context]((https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/platform/device_context.h#L37))
+After a new kind of Device is added, you should add a corresponding [DeviceContext](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/platform/device_context.h#L37) for it.
+
+```cpp
+class DeviceContext {
+ public:
+  virtual ~DeviceContext() {}
+  virtual Place GetPlace() const = 0;
+
+  virtual void Wait() const {}
+};
+```
+
+#### Implement new [OpKernel](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/operator.h#L351) for your Device.
+
+A detailed documentation can be found in [`new_op_and_kernel`](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/howto/dev/new_op_en.md)
+
+```cpp
+class OpKernelBase {
+ public:
+  /**
+   * ExecutionContext is the only parameter of Kernel Run function.
+   * Run will get input/output variables, state such as momentum and
+   * device resource such as CUDA stream, cublas handle, etc. from
+   * ExecutionContext. User should construct it before run the Operator.
+   */
+
+  virtual void Compute(const ExecutionContext& context) const = 0;
+
+  virtual ~OpKernelBase() = default;
+};
+
+template <typename T>
+class OpKernel : public OpKernelBase {
+ public:
+  using ELEMENT_TYPE = T;
+};
+```
+
+
+#### Register the OpKernel to framework
+
+After writing the components described above, we should register the kernel to the framework.
+
+We use `REGISTER_OP_KERNEL` to do the registration.
+
+```cpp
+REGISTER_OP_KERNEL(
+	op_type,
+	library_type,
+	place_type,
+	kernel0, kernel1, ...)
+```
+
+kernel0, kernel1 are kernels that have the same `op_type`, `library_type`, `place_type` but different `data_types`.
+
+take [`conv2d`]((https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/operators/conv_cudnn_op.cu.cc#L318)) as an example:
+
+	```cpp
+	REGISTER_OP_KERNEL(conv2d, CPU, paddle::platform::CPUPlace,
+    		paddle::operators::GemmConvKernel<paddle::platform::CPUDeviceContext, float>,
+    		paddle::operators::GemmConvKernel<paddle::platform::CPUDeviceContext, double>);
+    
+	REGISTER_OP_KERNEL(conv2d, CUDNN, ::paddle::platform::CUDAPlace,
+	       paddle::operators::CUDNNConvOpKernel<float>,
+	       paddle::operators::CUDNNConvOpKernel<double>);
+	```
+
+In the code above:
+
+ - `conv2d` is the type/name of the operator
+ - `CUDNN/CPU` is `library`
+ - `paddle::platform::CUDAPlace/CPUPlace` is `place`
+ - template parameter `float/double` on `CUDNNConvOpKernel<T>` is `data_type`.