title: ValueError: Metric 'cosine' not valid. Use sorted(sklearn.neighbors.VALID_METRICS['ball_tree'])
body: <p>I'm trying to use cosine metric with Ball_tree algorithm since my data is of high dimensions. e.g.</p>
  <pre><code>nbrs = NearestNeighbors(n_neighbors=2,algorithm='ball_tree',metric='cosine').fit(data_seed)
  </code></pre>
  <p>However i get the following error:</p>
  <pre><code>ValueError: Metric 'cosine' not valid. Use sorted(sklearn.neighbors.VALID_METRICS['ball_tree']) to get valid options. Metric can also be a callable function.
  </code></pre>
  <p>I don't understand why i cannot use cosine similarity with ball tree?</p>