code: def vstack(blocks, format=None, dtype=None):
      if not blocks:
          return np.array([])
      if any(sp.issparse(b) for b in blocks):
          return sp.vstack(blocks, format=format, dtype=dtype)
      else:
          return np.vstack(blocks)
  
file_path: eli5/eli5/utils.py
imports: import numpy
  from scipy import sparse
label_desc: Use image node_id 7 for calling a global function with example usage: vstack(blocks, format, dtype) and returns: np, sp, np