message: Accelerate CSR->Ell,Hybrid conversions on CUDA.
  + The previous grid dimensions for `initialize_zero_ell` were `stride *
  num_rows`, i.e. roughly the dense matrix dimension.
  + Using `max_nnz_per_row * num_rows` reduces significantly the amount of threads
  created which makes this kernel call more efficient (less useless thread
  creation).
diff: @@ -1137,7 +1137,8 @@ void convert_to_ell(std::shared_ptr<const CudaExecutor> exec,
  const auto num_rows = result->get_size()[0];
  const auto num_cols = result->get_size()[1];
  - const auto init_grid_dim = ceildiv(stride * num_rows, default_block_size);
  + const auto init_grid_dim =
  + ceildiv(max_nnz_per_row * num_rows, default_block_size);
  kernel::initialize_zero_ell<<<init_grid_dim, default_block_size>>>(
  max_nnz_per_row, stride, as_cuda_type(result_values),
  @@ -1471,7 +1472,7 @@ void convert_to_hybrid(std::shared_ptr<const CudaExecutor> exec,
  const auto max_nnz_per_row = result->get_ell_num_stored_elements_per_row();
  const auto num_rows = result->get_size()[0];
  const auto coo_num_stored_elements = result->get_coo_num_stored_elements();
  - auto grid_dim = ceildiv(stride * num_rows, default_block_size);
  + auto grid_dim = ceildiv(max_nnz_per_row * num_rows, default_block_size);
  kernel::initialize_zero_ell<<<grid_dim, default_block_size>>>(
  max_nnz_per_row, stride, as_cuda_type(ell_val), as_cuda_type(ell_col));
  