doc(gpu): add an example to use arrays on GPU

tfhe/docs/guides/run_on_gpu.md

@@ -256,3 +256,78 @@ fn main() {
 
 }
 ```
+
+## Array types
+
+It is possible to use array types on GPU, just as [on CPU](array.md). Here is an example showing how to do it:
+```rust
+use tfhe::{ConfigBuilder, set_server_key, ClearArray, ClientKey, CompressedServerKey};
+use tfhe::array::GpuFheUint32Array;
+use tfhe::prelude::*;
+
+fn main() {
+    let config = ConfigBuilder::default().build();
+
+    let cks = ClientKey::generate(config);
+    let compressed_server_key = CompressedServerKey::new(&cks);
+
+    let gpu_key = compressed_server_key.decompress_to_gpu();
+    set_server_key(gpu_key);
+
+    let num_elems = 4 * 4;
+    let clear_xs = (0..num_elems as u32).collect::<Vec<_>>();
+    let clear_ys = vec![1u32; num_elems];
+
+    // Encrypted 2D array with values
+    // [[  0,  1,  2,  3]
+    //  [  4,  5,  6,  7]
+    //  [  8,  9, 10, 11]
+    //  [ 12, 13, 14, 15]]
+    let xs = GpuFheUint32Array::try_encrypt((clear_xs.as_slice(), vec![4, 4]), &cks).unwrap();
+    // Encrypted 2D array with values
+    // [[  1,  1,  1,  1]
+    //  [  1,  1,  1,  1]
+    //  [  1,  1,  1,  1]
+    //  [  1,  1,  1,  1]]
+    let ys = GpuFheUint32Array::try_encrypt((clear_ys.as_slice(), vec![4, 4]), &cks).unwrap();
+
+    assert_eq!(xs.num_dim(), 2);
+    assert_eq!(xs.shape(), &[4, 4]);
+    assert_eq!(ys.num_dim(), 2);
+    assert_eq!(ys.shape(), &[4, 4]);
+
+    // Take a sub slice
+    //  [[ 10, 11]
+    //   [ 14, 15]]
+    let xss = xs.slice(&[2..4, 2..4]);
+    // Take a sub slice
+    //  [[  1,  1]
+    //   [  1,  1]]
+    let yss = ys.slice(&[2..4, 2..4]);
+
+    assert_eq!(xss.num_dim(), 2);
+    assert_eq!(xss.shape(), &[2, 2]);
+    assert_eq!(yss.num_dim(), 2);
+    assert_eq!(yss.shape(), &[2, 2]);
+
+    let r = &xss + &yss;
+
+    // Result is
+    //  [[ 11, 12]
+    //   [ 15, 16]]
+    let result: Vec<u32> = r.decrypt(&cks);
+    assert_eq!(result, vec![11, 12, 15, 16]);
+
+    // Clear 2D array with values
+    //  [[  10,  20]
+    //   [  30,  40]]
+    let clear_array = ClearArray::new(vec![10u32, 20u32, 30u32, 40u32], vec![2, 2]);
+    let r = &xss + &clear_array;
+
+    // Result is
+    //  [[ 20, 31]
+    //   [ 44, 55]]
+    let r: Vec<u32> = r.decrypt(&cks);
+    assert_eq!(r, vec![20, 31, 44, 55]);
+}
+```