Functions.cpp and MmBackward

1

Hi

I am trying to look at the gradients generated at the non-leaf/non-root nodes using an example in tutorials. The grad_fn function for one of the nodes in the created graph is MmBackward.

I searched Functions.cpp in generated folder and found this function:

variable_list MmBackward::apply(variable_list&& grads) {
  IndexRangeGenerator gen; 
  auto self_ix = gen.range(1);
  auto mat2_ix = gen.range(1);
  variable_list grad_inputs(gen.size());
  auto& grad = grads[0];
  auto self = self_.unpack();
  auto mat2 = mat2_.unpack();
  if (should_compute_output({ mat2_ix })) {
    auto grad_result = mm_mat2_backward(grad, self, mat2_sizes, mat2.strides(), 1);
    copy_range(grad_inputs, mat2_ix, grad_result);
  }
  if (should_compute_output({ self_ix })) {
    auto grad_result = mm_mat1_backward(grad, mat2, self, 1);
    copy_range(grad_inputs, self_ix, grad_result);
  }
  return grad_inputs;
}

Using gdb I tried to look at grad_inputs. I would like to ask two questions:

  1. Why is it of size 2?
  2. Why does it seem empty?
(gdb) p grad_inputs
$2 = {
  <std::_Vector_base<torch::autograd::Variable, std::allocator<torch::autograd::Variable> >> = {
    _M_impl = {
      <std::allocator<torch::autograd::Variable>> = {
        <__gnu_cxx::new_allocator<torch::autograd::Variable>> = {<No data fields>}, <No data fields>}, 
      members of std::_Vector_base<torch::autograd::Variable, std::allocator<torch::autograd::Variable> >::_Vector_impl: 
      _M_start = 0x7fdf8c000b10, 
      _M_finish = 0x7fdf8c000b20, 
      _M_end_of_storage = 0x7fdf8c000b20
    }
  }, <No data fields>}

(gdb) p *grad_inputs._M_impl._M_start
$8 = {
  <at::Tensor> = {
    impl_ = {
      target_ = 0x7fdfa91de340 <c10::UndefinedTensorImpl::_singleton>
    }
  }, <No data fields>}