code: def _gradient_y(img: torch.Tensor) -> torch.Tensor:
      if len(img.shape) != 4:
          raise AssertionError(img.shape)
      return img[:, :, :-1, :] - img[:, :, 1:, :]
  
file_path: kornia/kornia/losses/depth_smooth.py
imports: from __future__ import annotations
  import torch
  from torch import nn
label_desc: Use image node_id 2 for calling a global function with example usage: _gradient_y(img) and returns: unknown