Fix empty edge indices handling (#332)

* update

* fix empty

Author: rusty1s

torch_sparse/storage.py

@@ -146,7 +146,7 @@ def __init__(
         self._csr2csc = csr2csc
         self._csc2csr = csc2csr
 
-        if not is_sorted:
+        if not is_sorted and self._col.numel() > 0:
             idx = self._col.new_zeros(self._col.numel() + 1)
             idx[1:] = self.row()
             idx[1:] *= self._sparse_sizes[1]

torch_sparse/tensor.py

@@ -357,6 +357,8 @@ def numel(self) -> int:
             return self.nnz()
 
     def density(self) -> float:
+        if self.sparse_size(0) == 0 or self.sparse_size(1) == 0:
+            return 0.0
         return self.nnz() / (self.sparse_size(0) * self.sparse_size(1))
 
     def sparsity(self) -> float: