content: https://docs.pytorch.org/docs/stable/sparse.html
  
  * attribute
  
  Currently, one can acquire the COO format data only when the tensor instance is coalesced:
  s.indices()
  
  For acquiring the COO format data of an uncoalesced tensor, use torch.Tensor._values() and torch.Tensor._indices():
  
  
  * Uncoalesced sparse COO tensors
  
  
  >>> i = [[1, 1]]
  >>> v =  [3, 4]
  >>> s=torch.sparse_coo_tensor(i, v, (3,))
  >>> s
  tensor(indices=tensor([[1, 1]]),
         values=tensor(  [3, 4]),
         size=(3,), nnz=2, layout=torch.sparse_coo)
  
  coalesce会生成7
  