class_docstr: Lunar Multi-headed attention.
  See "Linformer: Self-Attention with Linear Complexity" for more details.
code: def apply_sparse_mask(
      attn_weights, tgt_len: int, src_len: int, bsz: int
  ):
      return attn_weights
  
imports: import math
  from typing import Dict, Optional, Tuple, Union
  import torch
  import torch.nn.functional
  from fairseq import utils
  from fairseq.incremental_decoding_utils import with_incremental_state
  from fairseq.modules.fairseq_dropout import FairseqDropout
  from torch import nn, Tensor
label_desc: Use image node_id 11 for calling the LunarMultiheadAttention obj's underlying member method code with example usage: obj.apply_sparse_mask(attn_weights, tgt_len, src_len, bsz) and returns: attn_weights