Turn all string indices into int indices, preserving ellipsis.

def _indices(self, indices):
    """Turn all string indices into int indices, preserving ellipsis."""
    if isinstance(indices, tuple):
      out = []
      dim = 0
      for i, index in enumerate(indices):
        if index is Ellipsis:
          out.append(index)
          dim = len(self.shape) - len(indices) + i
        else:
          out.append(self._get_index(dim, index))
        dim += 1
      return tuple(out)
    else:
      return self._get_index(0, indices)