wasi-nn: Support multiple TFLite models (#2002)

Remove restrictions:
- Only 1 WASM app at a time
- Only 1 model at a time
   - `graph` and `graph-execution-context` are ignored

Refer to previous document:
e8d718096d/core/iwasm/libraries/wasi-nn/README.md

build-scripts/runtime_lib.cmake

@@ -109,6 +109,13 @@ if (WAMR_BUILD_WASI_NN EQUAL 1)
         message("Tensorflow is already downloaded.")
     endif()
     set(TENSORFLOW_SOURCE_DIR "${WAMR_ROOT_DIR}/core/deps/tensorflow-src")
+
+    if (WASI_NN_ENABLE_GPU EQUAL 1)
+        # Tensorflow specific:
+        # * https://www.tensorflow.org/lite/guide/build_cmake#available_options_to_build_tensorflow_lite
+        set (TFLITE_ENABLE_GPU ON)
+    endif ()
+
     include_directories (${CMAKE_CURRENT_BINARY_DIR}/flatbuffers/include)
     include_directories (${TENSORFLOW_SOURCE_DIR})
     add_subdirectory(