Add include/ to .gitignore

Adds exp function to the Tensor class.

.gitignore

@@ -3,4 +3,3 @@ include/
 lib/
 lib64
 pyvenv.cfg
-

nn.py

@@ -102,7 +102,18 @@ class Tensor:
 
     tensor._back = back
     return tensor
-  
+
+  def exp(self):
+    tensor = Tensor(np.exp(self.value))
+    tensor._save(self)
+
+    def back(upstream):
+      a, = tensor._parents
+      return [ np.exp(a.value) ]
+
+    tensor._back = back
+    return tensor
+
   def _backprop(tensor, upstream):
     # Backprop through the tensor iff it has any parents.
     if tensor._parents is not None: