title: einsum on a sparse matrix
body: <p>It seems numpy's <code>einsum</code> function does not work with <code>scipy.sparse</code> matrices. Are there alternatives to do the sorts of things <code>einsum</code> can do with sparse matrices?</p>
  
  <p>In response to @eickenberg's answer: The particular einsum I'm wanting to is <code>numpy.einsum("ki,kj-&gt;ij",A,A)</code> - the sum of the outer products of the rows.</p>