Replace reference to variable by tensor (#530)

Author: shagunsodhani

beginner_source/ptcheat.rst

@@ -18,7 +18,7 @@ Neural Network API
 .. code-block:: python
 
     import torch.autograd as autograd         # computation graph
-    from torch.autograd import Variable       # variable node in computation graph
+    from torch import Tensor                  # tensor node in the computation graph
     import torch.nn as nn                     # neural networks
     import torch.nn.functional as F           # layers, activations and more
     import torch.optim as optim               # optimizers e.g. gradient descent, ADAM, etc.