docs(gpu): regroup gpu docs

docs(gpu): a small fix

tfhe/docs/configuration/gpu_acceleration/compressing_ciphertexts.md

@@ -0,0 +1,71 @@
+# Compressing ciphertexts
+
+This document explains how to compress ciphertexts using the GPU - even after homomorphic computations - just like on the [CPU](../../fhe-computation/data-handling/compress.md#compression-ciphertexts-after-some-homomorphic-computation).
+
+Compressing ciphertexts after computation using GPU is very similar to how it's done on the CPU. The following example shows how to compress and decompress a list containing 4 messages:
+
+* One 32-bits integer
+* One 64-bit integer
+* One Boolean
+* One 2-bit integer
+
+```rust
+use tfhe::prelude::*;
+use tfhe::shortint::parameters::{
+    COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS, PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS,
+};
+use tfhe::{
+    set_server_key, CompressedCiphertextList, CompressedCiphertextListBuilder, FheBool,
+    FheInt64, FheUint16, FheUint2, FheUint32,
+};
+
+fn main() {
+    let config =
+        tfhe::ConfigBuilder::with_custom_parameters(PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
+            .enable_compression(COMP_PARAM_GPU_MULTI_BIT_GROUP_4_MESSAGE_2_CARRY_2_KS_PBS)
+            .build();
+
+    let ck = tfhe::ClientKey::generate(config);
+    let compressed_server_key = tfhe::CompressedServerKey::new(&ck);
+    let gpu_key = compressed_server_key.decompress_to_gpu();
+
+    set_server_key(gpu_key);
+
+    let ct1 = FheUint32::encrypt(17_u32, &ck);
+
+    let ct2 = FheInt64::encrypt(-1i64, &ck);
+
+    let ct3 = FheBool::encrypt(false, &ck);
+
+    let ct4 = FheUint2::encrypt(3u8, &ck);
+
+    let compressed_list = CompressedCiphertextListBuilder::new()
+        .push(ct1)
+        .push(ct2)
+        .push(ct3)
+        .push(ct4)
+        .build()
+        .unwrap();
+
+    let serialized = bincode::serialize(&compressed_list).unwrap();
+
+    println!("Serialized size: {} bytes", serialized.len());
+
+    let compressed_list: CompressedCiphertextList = bincode::deserialize(&serialized).unwrap();
+
+    let a: FheUint32 = compressed_list.get(0).unwrap().unwrap();
+    let b: FheInt64 = compressed_list.get(1).unwrap().unwrap();
+    let c: FheBool = compressed_list.get(2).unwrap().unwrap();
+    let d: FheUint2 = compressed_list.get(3).unwrap().unwrap();
+
+    let a: u32 = a.decrypt(&ck);
+    assert_eq!(a, 17);
+    let b: i64 = b.decrypt(&ck);
+    assert_eq!(b, -1);
+    let c = c.decrypt(&ck);
+    assert!(!c);
+    let d: u8 = d.decrypt(&ck);
+    assert_eq!(d, 3);
+
+}
+```