[Bugfix] Remove unnecessary assert in from_scipy_sparse_matrix (#391)

python/dgl/graph_index.py

@@ -756,8 +756,6 @@ class GraphIndex(object):
         ----------
         adj : scipy sparse matrix
         """
-        assert isinstance(adj, (scipy.sparse.csr_matrix, scipy.sparse.coo_matrix)), \
-                "The input matrix has to be a SciPy sparse matrix."
         if not self.is_readonly():
             self.clear()
         num_nodes = max(adj.shape[0], adj.shape[1])