title: Finding which rows have all elements as zeros in a matrix with numpy
body: <p>I have a large <code>numpy</code> matrix <code>M</code>. Some of the rows of the matrix have all of their elements as zero and I need to get the indices of those rows. The naive approach I'm considering is to loop through each row in the matrix and then check each elements.</p>
  <p>What would be a better and a faster approach to accomplish this using <code>numpy</code>?</p>