Tests and docs for registering custom ONNX operators (#2416)

* Add tests, examples and documentation changes for custom ONNX operators registration mechanism

* Change snippet paths

* fix CoreThreadingTests.ReadNetwork - data race in ops_bridge

* Make TemplateExtension::Operation externally visible

* changes after review

* apply code format

* use std::int64_t

* forward declare get_attribute_value specializations

* introduce unregister_operator in onnx_importer

* onnx_custom_op - lock mem first then take a buffer

* func tests - create template_extension via make_so_pointer

* fix build with NGRAPH_ONNX_IMPORT_ENABLE=OFF

* remove exports from Operation and Extension

* Move multithreaded AddExtension test to different directory to it can be excluded when NGRAPH_ONNX_IMPORT_ENABLE=OFF

* Dont include Extension tests if ENABLE_MKL_DNN=OFF

* fix excluding onnx_reader tests

* include extension tests only if mkl is enabled

* add comment on empty blob

* use register_operator conditionally in template_extension

* fix docs after review

* create static library from onnx_custom_op

* add additional test for unregister_operator

* move model example after register step

* revert changes to unit tests

* update ngraphConfig.cmake.in header

* add headers to onnx_custom_op

* changes to docs CMakeLists

* remove redundant onnx_importer dependency

* remove extension directory from func tests

* make onnx_importer a component of ngraph package

* docs fixes

* update header of ngraph/cmake/share/ngraphConfig.cmake.in with ngraph_onnx_importer_FOUND

docs/IE_DG/Extensibility_DG/Custom_ONNX_Ops.md

@@ -0,0 +1,57 @@
+# Custom ONNX operators {#openvino_docs_IE_DG_Extensibility_DG_Custom_ONNX_Ops}
+
+ONNX importer provides mechanism to register custom ONNX operators based on predefined or user-defined nGraph operations.
+The function responsible for registering a new operator is called `ngraph::onnx_import::register_operator` and is defined in `onnx_import/onnx_utils.hpp`.
+
+## Registering custom ONNX operator based on predefined nGraph operations
+
+The steps below explain how to register a custom ONNX operator, for example, CustomRelu, in a domain called com.example.
+CustomRelu is defined as follows:
+```
+x >= 0 => f(x) = x * alpha
+x < 0  => f(x) = x * beta
+```
+where alpha, beta are float constants.
+
+1. Include headers:
+@snippet onnx_custom_op/main.cpp onnx_custom_op:headers
+
+2. Register the CustomRelu operator in the ONNX importer:
+@snippet onnx_custom_op/main.cpp onnx_custom_op:register_operator
+The `register_operator` function takes four arguments: op_type, opset version, domain, and a function object.
+The function object is a user-defined function that takes `ngraph::onnx_import::Node` as an input and based on that, returns a graph with nGraph operations.
+The `ngraph::onnx_import::Node` class represents a node in ONNX model. It provides functions to fetch input node(s) (`get_ng_inputs`), fetch attribute value (`get_attribute_value`) and many more (please refer to `onnx_import/core/node.hpp` for full class declaration).
+New operator registration must happen before the ONNX model is read, for example, if an ONNX model uses the 'CustomRelu' operator, `register_operator("CustomRelu", ...)` must be called before InferenceEngine::Core::ReadNetwork.
+Re-registering ONNX operators within the same process is supported. During registration of the existing operator, a warning is printed.
+
+The example below demonstrates an examplary model that requires previously created 'CustomRelu' operator:
+@snippet onnx_custom_op/main.cpp onnx_custom_op:model
+
+
+For a reference on how to create a graph with nGraph operations, visit [nGraph tutorial](../nGraphTutorial.md).
+For a complete list of predefined nGraph operators, visit [available operations sets](../../ops/opset.md).
+
+If operator is no longer needed, it can be unregistered by calling `unregister_operator`. The function takes three arguments `op_type`, `version`, and `domain`.
+@snippet onnx_custom_op/main.cpp onnx_custom_op:unregister_operator
+
+## Registering custom ONNX operator based on custom nGraph operations
+
+The same principles apply when registering custom ONNX operator based on custom nGraph operations.
+This example shows how to register custom ONNX operator based on `Operation` presented in [this tutorial](AddingNGraphOps.md), which is used in [TemplateExtension](Extension.md).
+@snippet extension.cpp extension:ctor
+
+Here, the `register_operator` function is called in Extension's constructor, which makes sure that it is called before InferenceEngine::Core::ReadNetwork (since InferenceEngine::Core::AddExtension must be called before a model with custom operator is read).
+
+The example below demonstrates how to unregister operator from Extension's destructor:
+@snippet extension.cpp extension:dtor
+Note that it is mandatory to unregister custom ONNX operator if it is defined in dynamic shared library.
+
+## Requirements for building with CMake
+
+Program that uses the `register_operator` functionality, requires (in addition to Inference Engine) `ngraph` and `onnx_importer` libraries.
+The `onnx_importer` is a component of `ngraph` package , so `find_package(ngraph REQUIRED COMPONENTS onnx_importer)` is sufficient to find both.
+The `ngraph` package exposes two variables (`${NGRAPH_LIBRARIES}` and `${ONNX_IMPORTER_LIBRARIES}`), which reference `ngraph` and `onnx_importer` libraries.
+Those variables need to be passed to the `target_link_libraries` command in the CMakeLists.txt file.
+
+See below CMakeLists.txt for reference:
+@snippet onnx_custom_op/CMakeLists.txt cmake:onnx_custom_op