Fix two usage errors: its vs it's (pytorch#1484)

Co-authored-by: holly1238 <[email protected]>

beginner_source/basics/autogradqs_tutorial.py

@@ -58,7 +58,7 @@
 # A function that we apply to tensors to construct computational graph is
 # in fact an object of class ``Function``. This object knows how to
 # compute the function in the *forward* direction, and also how to compute
-# it's derivative during the *backward propagation* step. A reference to
+# its derivative during the *backward propagation* step. A reference to
 # the backward propagation function is stored in ``grad_fn`` property of a
 # tensor. You can find more information of ``Function`` `in the
 # documentation <https://pytorch.org/docs/stable/autograd.html#function>`__.

beginner_source/basics/buildmodel_tutorial.py

@@ -67,7 +67,7 @@ def forward(self, x):
 
 ##############################################
 # We create an instance of ``NeuralNetwork``, and move it to the ``device``, and print 
-# it's structure.
+# its structure.
 
 model = NeuralNetwork().to(device)
 print(model)
@@ -119,7 +119,7 @@ def forward(self, x):
 # nn.Linear 
 # ^^^^^^^^^^^^^^^^^^^^^^
 # The `linear layer <https://pytorch.org/docs/stable/generated/torch.nn.Linear.html>`_
-# is a module that applies a linear transformation on the input using it's stored weights and biases.
+# is a module that applies a linear transformation on the input using its stored weights and biases.
 #
 layer1 = nn.Linear(in_features=28*28, out_features=20)
 hidden1 = layer1(flat_image)