code: def _generate_2d_grid(H, W):
      i = torch.arange(H)
      j = torch.arange(W)
      i, j = torch.meshgrid(i, j)
      return i, j
  
imports: import itertools
  import pytest
  import torch
  import xformers.components.attention.attention_patterns
  from xformers.components.attention.sparsity_config import BigBirdSparsityConfig, BSLongformerSparsityConfig, DenseSparsityConfig, FixedSparsityConfig, VariableSparsityConfig
label_desc: Use image node_id 2 for calling a global function with example usage: _generate_2d_grid(H, W) and returns: i,  j