docstr: Calculate the length of vector.
          
docstr_tok: ["Calculate","the","length","of","vector","."]
code: def l2_norm(self, vec):
      """Calculate the length of vector."""
      result = np.sqrt(np.sum(vec**2))
      return result
  
file_path: PaddleHub/modules/text/language_model/lda_news/semantic_matching.py
imports: import numpy
  from lda_news.vocab import OOV
label_desc: Use image node_id 2 for calling the SemanticMatching obj's underlying member method code with example usage: obj.l2_norm(vec) and returns: result