wasi-nn: Add support of wasi-nn as shared lib (#2310)

## Context

Currently, WAMR supports compiling iwasm with flag `WAMR_BUILD_WASI_NN`.
However, there are scenarios where the user might prefer having it as a shared library.

## Proposed Changes

Decouple wasi-nn context management by internally managing the context given
a module instance reference.

core/iwasm/libraries/wasi-nn/external/CMakeLists.txt

@@ -0,0 +1,58 @@
+# Copyright (C) 2019 Intel Corporation.  All rights reserved.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+cmake_minimum_required(VERSION 3.16)
+project(wasi-nn C CXX)
+
+set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+
+set(WAMR_ROOT_DIR ${CMAKE_CURRENT_LIST_DIR}/../../../../..)
+set(WASI_NN_ROOT_DIR ${CMAKE_CURRENT_LIST_DIR}/..)
+
+if(NOT CMAKE_BUILD_TYPE)
+  set(CMAKE_BUILD_TYPE Debug)
+endif()
+
+#### libvmlib ####
+# NOTE: we build vmlib as a shared library here so that it can be
+# shared between iwasm and native libraries.
+include(${WASI_NN_ROOT_DIR}/cmake/iwasm_helper.cmake)
+include(${WAMR_ROOT_DIR}/build-scripts/runtime_lib.cmake)
+
+add_library(vmlib SHARED ${WAMR_RUNTIME_LIB_SOURCE})
+
+# iwasm
+include(${SHARED_DIR}/utils/uncommon/shared_uncommon.cmake)
+set(RUNTIME_SOURCE_ALL
+  ${WAMR_ROOT_DIR}/product-mini/platforms/${WAMR_BUILD_PLATFORM}/main.c
+  ${UNCOMMON_SHARED_SOURCE}
+)
+
+add_executable(iwasm ${RUNTIME_SOURCE_ALL})
+target_link_libraries(iwasm vmlib -lpthread -lm -ldl)
+
+#### TensorFlow ####
+
+include(${WASI_NN_ROOT_DIR}/cmake/wasi_nn.cmake)
+
+#### WASI-NN ####
+
+include_directories(
+  ${WAMR_ROOT_DIR}/core/iwasm/include
+  ${WAMR_ROOT_DIR}/core/shared/utils
+  ${WAMR_ROOT_DIR}/core/shared/platform/linux
+)
+
+add_library(wasi-nn SHARED
+  ${WASI_NN_SOURCES}
+)
+
+# Add `get_native_lib` symbol
+target_compile_definitions(wasi-nn PUBLIC
+  WASI_NN_SHARED
+)
+
+target_link_libraries(wasi-nn
+  ${WASI_NN_LIBS}
+  vmlib
+)

core/iwasm/libraries/wasi-nn/external/README.md

@@ -0,0 +1,13 @@
+# wasi-nn as shared library
+
+Example on how to create libwasi-nn (external library) instead of embedding wasi-nn inside iwasm
+
+From folder `core/iwasm/libraries/wasi-nn/test`, build the test and run
+
+```sh
+../external/build/iwasm \
+    --dir=. \
+    --env="TARGET=cpu" \
+    --native-lib=../external/build/libwasi-nn.so \
+    test_tensorflow.wasm 
+```