title: scipy sparse matrix: remove the rows whose all elements are zero
body: <p>I have a sparse matrix which is transformed from sklearn tfidfVectorier. I believe that some rows are all-zero rows. I want to remove them. However, as far as I know, the existing built-in functions, e.g. nonzero() and eliminate_zero(), focus on zero entries, rather than rows. </p>
  
  <p>Is there any easy way to remove all-zero rows from a sparse matrix?</p>
  
  <p>Example:
  What I have now (actually in sparse format):</p>
  
  <pre><code>[ [0, 0, 0]
    [1, 0, 2]
    [0, 0, 1] ]
  </code></pre>
  
  <p>What I want to get:</p>
  
  <pre><code>[ [1, 0, 2]
    [0, 0, 1] ]
  </code></pre>