Document hybrid sparse tensors.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Author: ezyang

docs/source/sparse.rst

@@ -21,10 +21,25 @@ by providing these two tensors, as well as the size of the sparse tensor
  >>> i = torch.LongTensor([[0, 1], [2, 0]])
  >>> v = torch.FloatTensor([3, 4])
  >>> torch.sparse.FloatTensor(i, v, torch.Size([2,3])).to_dense()
- 0 0 3
- 4 0 0
+
+ 0 0 3
+ 4 0 0
  [torch.FloatTensor of size 2x2]
 
+You can also construct hybrid sparse tensors, where only the first n
+dimensions are sparse, and the rest of the dimensions are dense.
+
+ >>> i = torch.LongTensor([[2, 4]])
+ >>> v = torch.FloatTensor([[1, 3], [5, 7]])
+ >>> torch.sparse.FloatTensor(i, v).to_dense()
+
+ 0 0
+ 0 0
+ 1 3
+ 0 0
+ 5 7
+ [torch.FloatTensor of size 5x2]
+
 An empty sparse tensor can be constructed by specifying its size:
 
  >>> torch.sparse.FloatTensor(2, 3)