code: def _gradient_x(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 1 for calling a global function with example usage: _gradient_x(img) and returns: unknown