Fix docs: masked_fill_ takes a value, not a tensor. (pytorch#1663)

Author: gchanan

torch/_tensor_docs.py

@@ -827,7 +827,7 @@ def callable(a, b) -> number
 
 Args:
  mask (ByteTensor): The binary mask
- value (Tensor): The value to fill
+ value (float): The value to fill
 """)
 
 add_docstr(torch._C.FloatTensorBase.masked_select,