title: How to join 3 numpy csr sparse matrix into 1 matrix
body: <p>I have 3 numpy csr matrixes a, b, c of shape <br/>
  (69918, 7559) <br/>
  (17480, 7559) <br/>
  (21850, 7559)</p>
  
  <p>these are the return of numpy.hstack((some matrices ....)).to_csr()</p>
  
  <p>I want to join them and convert them into shape <br/>
  (69918+17480+21850, 7559) <br/></p>
  
  <p>How should I do it.
  I tried numpy.vstack function but that converts it into (3, 1) shape  </p>