feat(gpu): use gemm ks in HL ops

[andrei-stoian-zama]

• Uses GEMM-based KS in integer ops when large batches of LWEs need to be keyswitched
• Implements GEMM KS with non-trivial indexes
• Improves KS GPU bench and KS GPU test

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This change is 

[IceTDrinker]

nitpick comments on the core non cuda part (I did not read that one), thanks!

@IceTDrinker reviewed 7 of 22 files at r1, all commit messages.
Reviewable status: 7 of 22 files reviewed, 13 unresolved discussions (waiting on @agnesLeroy and @soonum)

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tfhe/src/core_crypto/gpu/algorithms/lwe_keyswitch.rs line 79 at r1 (raw file):

    let mut ks_tmp_buffer: *mut ffi::c_void = std::ptr::null_mut();

    let num_lwes_to_ks = min(

it's possible to only partially keyswitch an input ?

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tfhe/src/core_crypto/gpu/algorithms/lwe_keyswitch.rs line 84 at r1 (raw file):

    );

    assert_eq!(input_indexes.len, output_indexes.len);

error message could be welcome

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tfhe-benchmark/benches/core_crypto/ks_bench.rs line 458 at r1 (raw file):

                            let input_ks_list = LweCiphertextList::from_container(
                                input_ct_list.into_container(),
                                big_lwe_sk.lwe_dimension().to_lwe_size(),

same nits as tests will apply here about key dimensions and ciphertext counts and places where things get collected/transformed into vecs

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 118 at r1 (raw file):

        msg = msg.wrapping_sub(Scalar::ONE);
        for test_idx in 0..NB_TESTS {
            let num_blocks = test_idx * test_idx * 3 + 1;

are those magic numbers ? or could those be randomly chosen ?

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 139 at r1 (raw file):

                &mut rsc.encryption_random_generator,
            );
            let input_ks_list = LweCiphertextList::from_container(

why is this required ? it looks like it's just recreating the input_ct_list ?

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 149 at r1 (raw file):

            let output_ct_list = LweCiphertextList::new(
                Scalar::ZERO,
                lwe_sk.lwe_dimension().to_lwe_size(),

nit: prefer using the compute key (here ksk) dimension that will be used, tends to help with local reasoning

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 150 at r1 (raw file):

                Scalar::ZERO,
                lwe_sk.lwe_dimension().to_lwe_size(),
                LweCiphertextCount(num_blocks),

nit: use the input.lwe_ciphertext_count(), same for local reasoning

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 151 at r1 (raw file):

                lwe_sk.lwe_dimension().to_lwe_size(),
                LweCiphertextCount(num_blocks),
                ciphertext_modulus,

nit: again use the output modulus of the compute key (ksk)

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 173 at r1 (raw file):

            };
            let lwe_indexes_usize = (0..num_blocks).collect_vec();
            let mut lwe_indexes = lwe_indexes_usize.iter().collect_vec();

let mut lwe_indexes = lwe_indexes_usize.clone();

?

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 182 at r1 (raw file):

            }

            if num_blocks_to_ks < num_blocks {

no need to do the check you can always do

lwe_indexes = lwe_indexes[..num_blocks_to_ks];

I believe since the num to ks should always be <= num_blocks ?

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 184 at r1 (raw file):

            if num_blocks_to_ks < num_blocks {
                lwe_indexes = lwe_indexes[0..num_blocks_to_ks].to_vec();
                lwe_indexes_out = lwe_indexes_out[0..num_blocks_to_ks].to_vec();

I don't think you need the to_vec

can take a slice like

lwe_indexes = &lwe_indexes[..num_blocks_to_ks];

the whole thing above can be put in the iter below will give an example

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 187 at r1 (raw file):

            }

            let h_lwe_indexes: Vec<Scalar> = lwe_indexes

lwe_indexes.iter().take(num_blocks_to_ks).map(...)

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tfhe/src/core_crypto/gpu/algorithms/test/lwe_keyswitch.rs line 216 at r1 (raw file):

            for i in 0..num_blocks_to_ks {
                ref_vec[*lwe_indexes_out[i]] =
                    round_decode(*plaintext_list.get(*lwe_indexes[i]).0, delta); // % msg_modulus;

comment can be removed I think ?