minor spelling tweaks (#349)

* minor spelling tweaks

* Update CONTRIBUTORS.md

tutorials/models/2_small_graph/3_tree-lstm.py

@@ -204,7 +204,7 @@ class TreeLSTMCell(nn.Module):
 # *simultaneously*.
 #
 # In the case of Tree-LSTM, messages start from leaves of the tree, and
-# propogate/processed upwards until they reach the roots. A visulization
+# propagate/processed upwards until they reach the roots. A visualization
 # is as follows:
 #
 # .. figure:: https://i.loli.net/2018/11/09/5be4b5d2df54d.gif

tutorials/models/3_generative_model/5_dgmg.py

@@ -198,7 +198,7 @@ def forward_train(self, actions):
         - Contains a_1, ..., a_T described above
     - self.prepare_for_train()
         - Initializes self.action_step to be 0, which will get
-          incremented by 1 everytime it is called.
+          incremented by 1 every time it is called.
         - Initializes objects recording log p(a_t|a_1,...a_{t-1})
 
     Returns