Wasi-nn WinML backend returns output size for get_output. (#8745)

This change fixes an issue that get_output always returns 0.

crates/wasi-nn/src/backend/winml.rs

@@ -3,7 +3,7 @@
 use super::{BackendError, BackendExecutionContext, BackendFromDir, BackendGraph, BackendInner};
 use crate::wit::types::{ExecutionTarget, GraphEncoding, Tensor};
 use crate::{ExecutionContext, Graph};
-use std::{fs::File, io::Read, path::Path};
+use std::{fs::File, io::Read, mem::size_of, path::Path};
 use windows::core::{ComInterface, HSTRING};
 use windows::Storage::Streams::{
     DataWriter, InMemoryRandomAccessStream, RandomAccessStreamReference,
@@ -134,7 +134,6 @@ impl BackendExecutionContext for WinMLExecutionContext {
         let output_name = self.session.Model()?.OutputFeatures()?.GetAt(index)?;
         let output_name_hstring = output_name.Name()?;
 
-        // Print results.
         let vector_view = self
             .result
             .as_ref()
@@ -144,14 +143,15 @@ impl BackendExecutionContext for WinMLExecutionContext {
             .cast::<TensorFloat>()?
             .GetAsVectorView()?;
         let output: Vec<f32> = vector_view.into_iter().collect();
+        let len_to_copy = output.len() * size_of::<f32>();
         unsafe {
-            destination.copy_from_slice(std::slice::from_raw_parts(
+            destination[..len_to_copy].copy_from_slice(std::slice::from_raw_parts(
                 output.as_ptr() as *const u8,
-                output.len() * 4,
+                len_to_copy,
             ));
         }
 
-        Ok(0)
+        Ok(len_to_copy as u32)
     }
 }