add comments for grouped host code

example/52_flash_atten_bias/grouped_multihead_attention_bias_backward_v2.cpp

@@ -767,12 +767,13 @@ int run(int argc, char* argv[])
             auto v_g_o_n = v_g_n_os[i].Transpose({0, 2, 1});
             ref_gemm0_grad_invoker.Run(RefGemm0GradArg{
                 ygrad_g_m_o, v_g_o_n, pgrad_drop_g_m_n, PassThrough{}, PassThrough{}, Scale{1.f}});
+            // dP = dP_dropout x Z
             auto ref_dropout         = ReferenceDropoutInstance{};
             auto ref_dropout_invoker = ref_dropout.MakeInvoker();
             auto ref_dropout_argment = ref_dropout.MakeArgument(
                 z_g_m_ns[i], pgrad_drop_g_m_n, pgrad_g_m_n, p_dropout_in_16bits, rp_dropout);
             ref_dropout_invoker.Run(ref_dropout_argment);
-
+            // dS_i_j = P_i_j .* (dP_i_j - dY_i dot Y_i)
             sgrad_g_m_n.ForEach([&](auto& self, auto idx_gmn) {
                 float ygrad_dot_y = 0;
                 for(int o = 0; o < O; o++)
@@ -786,12 +787,14 @@ int run(int argc, char* argv[])
                     ck::type_convert<AccDataType>(p_g_m_ns[i](idx_gmn)) *
                     (ck::type_convert<AccDataType>(pgrad_g_m_n(idx_gmn)) - ygrad_dot_y));
             });
-
+            // dV = P_drop^T * dY
             auto p_drop_g_n_m = p_drop_g_m_ns[i].Transpose({0, 2, 1});
             ref_gemm1_grad_invoker.Run(RefGemm1GradArg{
                 p_drop_g_n_m, ygrad_g_m_o, vgrad_g_n_o, PassThrough{}, PassThrough{}, Scale{1.0f}});
+            // dQ = alpha * dS * K
             ref_gemm1_grad_invoker.Run(RefGemm1GradArg{
                 sgrad_g_m_n, k_g_n_ks[i], qgrad_g_m_k, PassThrough{}, PassThrough{}, Scale{alpha}});
+            // dK = alpha * dS^T * Q
             auto sgrad_g_n_m = sgrad_g_m_n.Transpose({0, 2, 1});
             ref_gemm1_grad_invoker.Run(RefGemm1GradArg{
                 sgrad_g_n_m, q_g_m_ks[i], kgrad_g_n_k, PassThrough{}, PassThrough{}, Scale{alpha}});