Add an initial wasi-nn implementation for Wasmtime (#2208)

* Add an initial wasi-nn implementation for Wasmtime

This change adds a crate, `wasmtime-wasi-nn`, that uses `wiggle` to expose the current state of the wasi-nn API and `openvino` to implement the exposed functions. It includes an end-to-end test demonstrating how to do classification using wasi-nn:
 - `crates/wasi-nn/tests/classification-example` contains Rust code that is compiled to the `wasm32-wasi` target and run with a Wasmtime embedding that exposes the wasi-nn calls
 - the example uses Rust bindings for wasi-nn contained in `crates/wasi-nn/tests/wasi-nn-rust-bindings`; this crate contains code generated by `witx-bindgen` and eventually should be its own standalone crate

* Test wasi-nn as a CI step

This change adds:
 - a GitHub action for installing OpenVINO
 - a script, `ci/run-wasi-nn-example.sh`, to run the classification example

crates/wasi-nn/src/ctx.rs

@@ -0,0 +1,125 @@
+//! Implements the base structure (i.e. [WasiNnCtx]) that will provide the implementation of the
+//! wasi-nn API.
+use crate::r#impl::UsageError;
+use crate::witx::types::{Graph, GraphExecutionContext};
+use openvino::InferenceError;
+use std::cell::RefCell;
+use std::collections::HashMap;
+use std::hash::Hash;
+use thiserror::Error;
+use wiggle::GuestError;
+
+/// Possible errors for interacting with [WasiNnCtx].
+#[derive(Debug, Error)]
+pub enum WasiNnError {
+    #[error("guest error")]
+    GuestError(#[from] GuestError),
+    #[error("openvino error")]
+    OpenvinoError(#[from] InferenceError),
+    #[error("usage error")]
+    UsageError(#[from] UsageError),
+}
+
+pub(crate) type WasiNnResult<T> = std::result::Result<T, WasiNnError>;
+
+pub struct Table<K, V> {
+    entries: HashMap<K, V>,
+    next_key: u32,
+}
+
+impl<K, V> Default for Table<K, V> {
+    fn default() -> Self {
+        Self {
+            entries: HashMap::new(),
+            next_key: 0,
+        }
+    }
+}
+
+impl<K, V> Table<K, V>
+where
+    K: Eq + Hash + From<u32> + Copy,
+{
+    pub fn insert(&mut self, value: V) -> K {
+        let key = self.use_next_key();
+        self.entries.insert(key, value);
+        key
+    }
+
+    pub fn remove(&mut self, key: K) -> Option<V> {
+        self.entries.remove(&key)
+    }
+
+    pub fn get(&self, key: K) -> Option<&V> {
+        self.entries.get(&key)
+    }
+
+    pub fn get_mut(&mut self, key: K) -> Option<&mut V> {
+        self.entries.get_mut(&key)
+    }
+
+    pub fn len(&self) -> usize {
+        self.entries.len()
+    }
+
+    fn use_next_key(&mut self) -> K {
+        let current = self.next_key;
+        self.next_key += 1;
+        K::from(current)
+    }
+}
+
+pub struct ExecutionContext {
+    pub(crate) graph: Graph,
+    pub(crate) request: openvino::InferRequest,
+}
+
+impl ExecutionContext {
+    pub(crate) fn new(graph: Graph, request: openvino::InferRequest) -> Self {
+        Self { graph, request }
+    }
+}
+
+/// Capture the state necessary for calling into `openvino`.
+pub struct Ctx {
+    pub(crate) core: openvino::Core,
+    pub(crate) graphs: Table<Graph, (openvino::CNNNetwork, openvino::ExecutableNetwork)>,
+    pub(crate) executions: Table<GraphExecutionContext, ExecutionContext>,
+}
+
+impl Ctx {
+    /// Make a new `WasiNnCtx` with the default settings.
+    pub fn new() -> WasiNnResult<Self> {
+        Ok(Self {
+            core: openvino::Core::new(None)?,
+            graphs: Table::default(),
+            executions: Table::default(),
+        })
+    }
+}
+
+/// This structure provides the Rust-side context necessary for implementing the wasi-nn API. At the
+/// moment, it is specialized for a single inference implementation (i.e. OpenVINO) but conceivably
+/// this could support more than one backing implementation.
+pub struct WasiNnCtx {
+    pub(crate) ctx: RefCell<Ctx>,
+}
+
+impl WasiNnCtx {
+    /// Make a new `WasiNnCtx` with the default settings.
+    pub fn new() -> WasiNnResult<Self> {
+        Ok(Self {
+            ctx: RefCell::new(Ctx::new()?),
+        })
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn instantiate() {
+        WasiNnCtx::new().unwrap();
+    }
+}