Adding custom_module/sync/ & custom_module/async/ samples.
These show how to interact with tensor I/O when using either
synchronous or asynchronous custom module calls.

samples/custom_module/sync/README.md

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+# Synchronous tensor I/O custom module sample
+
+This sample expects that you've already produced a working version of the
+[basic sample](/samples/custom_module/basic/) (including compiler installation
+and CMake setup).
+
+This sample demonstrates adding custom modules callable from compiler-produced
+programs that take and return `tensor` types. By default custom calls are
+treated as blocking operations that synchronize with the underlying device to
+ensure all passed `tensor` buffer views are host coherent and it's assumed that
+any returned `tensor` buffer views are ready for use when the call returns.
+
+This approach is the easiest to integrate and looks similar to classic ML
+frameworks custom calls. There are many significant performance implications of
+using this approach, though, and synchronous calls should only be used when
+no asynchronous approach is possible. See the
+[async tensor](/samples/custom_module/async/) sample for how to define
+custom calls that work asynchronously.
+
+## Instructions
+
+1. Compile the [example module](./test/example.mlir) to a .vmfb file:
+
+    ```
+    iree-compile --iree-hal-target-backends=llvm-cpu samples/custom_module/sync/test/example.mlir -o=/tmp/example.vmfb
+    ```
+
+2. Build the `iree_samples_custom_module_sync_run` CMake target :
+
+    ```
+    cmake -B ../iree-build/ -DCMAKE_BUILD_TYPE=RelWithDebInfo . \
+        -DCMAKE_C_FLAGS=-DIREE_VM_EXECUTION_TRACING_FORCE_ENABLE=1
+    cmake --build ../iree-build/ --target iree_samples_custom_module_sync_run
+    ```
+    (here we force runtime execution tracing for demonstration purposes)
+
+    [See here](https://iree-org.github.io/iree/building-from-source/getting-started/)
+    for general instructions on building using CMake.
+
+3. Run the example program to call the main function:
+
+   ```
+   ../iree-build/samples/custom_module/sync/custom-module-sync-run \
+       /tmp/example.vmfb example.main
+   ```