class_docstr: Torch autograd Function wrapper for packed_to_padded C++/CUDA implementations.
code: def backward(ctx, grad_output):
      grad_output = grad_output.contiguous()
      first_idxs = ctx.saved_tensors[0]
      num_inputs = ctx.num_inputs
      grad_input = _C.padded_to_packed(
          grad_output, first_idxs, num_inputs
      )
      return grad_input, None, None
  
file_path: pytorch3d/pytorch3d/ops/packed_to_padded.py
imports: import torch
  from pytorch3d import _C
  from torch.autograd import Function
  from torch.autograd.function import once_differentiable
label_desc: Use image node_id 2 for calling the _PackedToPadded obj's underlying member method code with example usage: obj.backward(ctx, grad_output) and returns: grad_input,  None,  None