Function ggml_flash_attn_back

subprojects/llama.cpp/ggml/src/ggml.c:5334–5401

// ggml_flash_attn_back

struct ggml_tensor * ggml_flash_attn_back(
        struct ggml_context * ctx,
        struct ggml_tensor  * q,
        struct ggml_tensor  * k,
        struct ggml_tensor  * v,
        struct ggml_tensor  * d,
        bool                  masked) {
    GGML_ABORT("TODO: adapt to ggml_flash_attn_ext() changes");

    GGML_ASSERT(ggml_can_mul_mat(k, q));
    // TODO: check if vT can be multiplied by (k*qT)

    // d shape [D,N,ne2,ne3]
    // q shape [D,N,ne2,ne3]
    // k shape [D,M,kvne2,ne3]
    // v shape [M,D,kvne2,ne3]

    const int64_t     D = q->ne[0];
    const int64_t     N = q->ne[1];
    const int64_t     M = k->ne[1];
    const int64_t   ne2 = q->ne[2];
    const int64_t   ne3 = q->ne[3];
    const int64_t kvne2 = k->ne[2];

    GGML_ASSERT(k->ne[0] == D);
    GGML_ASSERT(v->ne[0] == M);
    GGML_ASSERT(v->ne[1] == D);
    GGML_ASSERT(d->ne[0] == D);
    GGML_ASSERT(d->ne[1] == N);
    GGML_ASSERT(k->ne[2] == kvne2);
    GGML_ASSERT(k->ne[3] == ne3);
    GGML_ASSERT(v->ne[2] == kvne2);
    GGML_ASSERT(v->ne[3] == ne3);
    GGML_ASSERT(d->ne[2] == ne2);
    GGML_ASSERT(d->ne[3] == ne3);

    GGML_ASSERT(ne2 % kvne2 == 0);

    // store gradients of q, k and v as continuous tensors concatenated in result.
    // note: v and gradv are actually transposed, i.e. v->ne[0] != D.
    const int64_t elem_q = ggml_nelements(q);
    const int64_t elem_k = ggml_nelements(k);
    const int64_t elem_v = ggml_nelements(v);

    enum ggml_type result_type = GGML_TYPE_F32;
    GGML_ASSERT(ggml_blck_size(result_type) == 1);
    const size_t tsize = ggml_type_size(result_type);

    const size_t offs_q = 0;
    const size_t offs_k = offs_q + GGML_PAD(elem_q * tsize, GGML_MEM_ALIGN);
    const size_t offs_v = offs_k + GGML_PAD(elem_k * tsize, GGML_MEM_ALIGN);
    const size_t end    = offs_v + GGML_PAD(elem_v * tsize, GGML_MEM_ALIGN);

    const size_t nelements = (end + tsize - 1)/tsize;

    struct ggml_tensor * result = ggml_new_tensor_1d(ctx, GGML_TYPE_F32, nelements);

    int32_t masked_i = masked ? 1 : 0;