Grammar fixes in Blitz tutorials (pytorch#255)

* fix grammar

* fix grammar

Author: orrblue

beginner_source/blitz/autograd_tutorial.py

@@ -31,7 +31,7 @@
 To prevent tracking history (and using memory), you can also wrap the code block
 in ``with torch.no_grad():``. This can be particularly helpful when evaluating a
 model because the model may have trainable parameters with `requires_grad=True`,
-but we don't need the gradients.
+but for which we don't need the gradients.
 
 There’s one more class which is very important for autograd
 implementation - a ``Function``.

beginner_source/blitz/tensor_tutorial.py

@@ -48,7 +48,7 @@
 print(x)
 
 ###############################################################
-# or create a tensor basing on existing tensor. These methods
+# or create a tensor based on an existing tensor. These methods
 # will reuse properties of the input tensor, e.g. dtype, unless
 # new values are provided by user