Add hello world example for C++ torchvision (#2003)

* Add hello world example for C++ torchvision

* README updates and mention the hello_world example

README.rst

@@ -82,8 +82,10 @@ Once installed, the library can be accessed in cmake (after properly configuring
 	find_package(TorchVision REQUIRED)
 	target_link_libraries(my-target PUBLIC TorchVision::TorchVision)
 
-The ``TorchVision`` package will also automatically look for the ``Torch`` and ``pybind11`` packages and add them as dependencies to ``my-target``,
-so make sure that they are also available to cmake via the ``CMAKE_PREFIX_PATH``.
+The ``TorchVision`` package will also automatically look for the ``Torch`` package and add it as a dependency to ``my-target``,
+so make sure that it is also available to cmake via the ``CMAKE_PREFIX_PATH``.
+
+For an example setup, take a look at ``examples/cpp/hello_world``.
 
 Documentation
 =============

examples/cpp/hello_world/CMakeLists.txt

+cmake_minimum_required(VERSION 3.10)
+project(hello-world)
+
+# The first thing do is to tell cmake to find the TorchVision library.
+# The package pulls in all the necessary torch libraries,
+# so there is no need to also add `find_package(Torch)` here.
+find_package(TorchVision REQUIRED)
+
+add_executable(hello-world main.cpp)
+
+# We now need to link the TorchVision library to our executable.
+# We can do that by using the TorchVision::TorchVision target,
+# which also adds all the necessary torch dependencies.
+target_link_libraries(hello-world TorchVision::TorchVision)

examples/cpp/hello_world/README.rst

+Hello World!
+============
+
+This is a minimal example of getting TorchVision to work in C++ with CMake.
+
+
+In order to successfully compile this example, make sure you have both ``LibTorch`` and
+``TorchVision`` installed.
+Once both dependencies are sorted, we can start the CMake fun:
+
+1) Create a ``build`` directory inside the current one.
+2) from within the ``build`` directory, run the following commands:
+    - | ``cmake -DCMAKE_PREFIX_PATH="<PATH_TO_LIBTORCH>;<PATH_TO_TORCHVISION>" ..``
+      | where ``<PATH_TO_LIBTORCH>`` and ``<PATH_TO_TORCHVISION>`` are the paths to the libtorch and torchvision installations.
+    - ``cmake --build .``
+
+| That's it!
+| You should now have a ``hello-world`` executable in your ``build`` folder.
+ Running it will output a (fairly long) tensor of random values to your terminal.
\ No newline at end of file

examples/cpp/hello_world/main.cpp

+#include <iostream>
+
+#include <torchvision/models/resnet.h>
+
+int main()
+{
+  auto model = vision::models::ResNet18();
+  model->eval();
+
+  // Create a random input tensor and run it through the model.
+  auto in = torch::rand({1, 3, 10, 10});
+  auto out = model->forward(in);
+
+  std::cout << out;
+}