[doc] Add documentation for external backends

Author: tlepley-cadence

docs/Backends.md

@@ -194,3 +194,14 @@ optimizations to take place.
 The `tools/ClassGen/Backends/CPU/CPUSpecificNodes.h` and
 `tools/ClassGen/Backends/CPU/CPUSpecificInstrs.h` files are included in
 `tools/ClassGen/NodeGen.cpp` and `tools/ClassGen/InstrGen.cpp`, respectively.
+
+
+#### External backends
+
+External backends can be added to Glow without changing the Glow build infrastructure.
+
+An external backend is provided as a single source directory. It can then be developped in a separate source management repository.
+
+The external backend mechanism is for instance convenient for adding closed source backends to Glow.
+
+The structure of external backends is defined [here](https://github.com/pytorch/glow/blob/master/docs/ExternalBackend.md).

docs/ExternalBackend.md

@@ -0,0 +1,128 @@
+## External backends
+
+An external backend can be added to Glow without changing the Glow build infrastructure. An external backend must be provided as a single source directory, and then can be developped in a separate source management repository.
+
+### External backend structure
+
+An external backend must have the following structure:
+```
+`-- <Name>
+    |-- CMakeLists.txt
+    |-- Backends
+    |   |-- CMakeLists.txt
+    |   |-- <main backend sources>
+    |-- ClassGen
+    |   |-- CMakeLists.txt
+    |   |-- <ClassGen sources>
+    `-- Tests
+        |-- CMakeLists.txt
+        |-- <Tests sources>
+```
+
+To be integrated into Glow and compiled, the backend directory must be placed in the `externalbackends/` sub-directory of the Glow project.
+
+
+### Backend structure details
+
+#### Defined cmake variables
+
+When an external backend is detected and activated, the following CMake variable is defined:
+
+- `EXTERNAL_BACKEND_NAME` : name derived from the external backend directory name. Can be used by the backend to name libraries for instance.
+
+
+There is the possibility to disable an detected external backend by setting the following CMake define to `OFF`:
+
+- `GLOW_WITH_<BACKEND DIR NAME>`
+
+
+#### C++ define
+
+When an external backend is activated, Glow adds a global C++ compiler define that can be used to enclose backend specific code in the generic Glow source code, based on the activation of this external backend:
+
+- `GLOW_WITH_<BACKEND DIR NAME>`
+
+
+#### Backend Core : Mandatory
+
+The core of the external backend is mandatory and located in `<Name>/Backends/`. It must contain a `CMakeLists.txt` file that will ensure the correct compilation of the backend and the good integration with Glow.
+
+The external backend must respect the same interface as standard backends.
+It can be summarized the following way:
+
+- Backend main class that inherits from the Glow `Backend` class. For example, class `<Name>Backend` in library `${EXTERNAL_BACKEND_NAME}`.
+- Backend factory registration in a backend specific factory library. For example, calling `REGISTER_GLOW_BACKEND_FACTORY(<Name>Factory, <Name>Backend, <Name>)` in library `${EXTERNAL_BACKEND_NAME}Factory`.
+
+
+The external backend is declared to Glow by setting the CMake `linked_factories` variable in the `<Name>/Backends/CMakeLists.txt`. Example:`set(linked_factories ${linked_factories} <Name>Factory PARENT_SCOPE)`
+
+For more details, please refer to the general Glow backend documentation.
+
+#### Top level CMake configuration file
+
+Global configuration of the backend can be specified in a top level `<Name>/CMakeLists.txt`.
+
+This file is optional.
+
+#### Backend specific nodes and instructions
+
+An external backend can optionally declare specific Glow nodes and instructions.
+In this case, the `<Name>/ClassGen/CMakeLists.txt` must be provided and backend specific nodes and instructions declaration headers declared.
+
+CMake example:
+```
+set(VERIF_FILENAME <Name>SpecificNodesVerification.h)
+configure_file(${VERIF_FILENAME}
+               ${GLOW_BINARY_DIR}/glow/${VERIF_FILENAME} COPYONLY)
+
+set(VERIF_FILENAME <Name>SpecificInstrsVerification.h)
+configure_file(${VERIF_FILENAME}
+               ${GLOW_BINARY_DIR}/glow/${VERIF_FILENAME} COPYONLY)
+```
+
+These header files must be included in the generic glow code, respectivelly in `tools/ClassGen/NodeGen.cpp` and `tools/ClassGen/InstrGen.cpp`, at the end of the `main` function like standard backends.It is advised to enclose the backend specific include into `#ifdef GLOW_WITH_<BACKEND DIR NAME>`.
+
+Example for specific nodes:
+```
+int main(int argc, char **argv) {
+  // <generic glow code>
+
+#include "Backends/CPU/CPUSpecificNodes.h"
+#include "Backends/OpenCL/OpenCLSpecificNodes.h"
+
+#ifdef GLOW_WITH_<NAME>
+# include "<Name>/ClassGen/<Name>SpecificNodes.h"
+#endif
+
+  return 0;
+}
+
+```
+
+
+#### Backend specific tests
+
+Backend specific tests can be added. In this case, the `<Name>/Tests/CMakeLists.txt` must be provided. The Backend tests must be compiled as a library and registered to glow with the `add_glow_test` cmake function.
+
+CMake example:
+```
+add_executable(<Name>Test
+		 <test sources>)
+
+target_link_libraries(<Name>Test
+                      PRIVATE
+                        Backends
+                        ExecutionEngine
+                        Graph
+                        Optimizer
+                        gtest
+                        TestMain)
+
+add_glow_test(<name>Test ${GLOW_BINARY_DIR}/tests/<name>Test --gtest_output=xml:Xtensa<name>Test.xml)`
+
+```
+
+
+For details, please refer to the general Glow testing documentation.
+
+