Fix typos

Thank you @phiresky for pointing these out. fixed.

imagernn/solver.py

@@ -10,7 +10,7 @@ class Solver:
   """
   def __init__(self):
     self.step_cache_ = {} # might need this
-    self.step_cache_2 = {} # might need this
+    self.step_cache2_ = {} # might need this
 
   def step(self, batch, model, cost_function, **kwargs):
     """ 
@@ -34,7 +34,7 @@ def step(self, batch, model, cost_function, **kwargs):
         if not u in self.step_cache_: 
           self.step_cache_[u] = np.zeros(model[u].shape)
           if solver == 'adadelta':
-            self.step_cache_2[u] = np.zeros(model[u].shape) # adadelta needs one more cache
+            self.step_cache2_[u] = np.zeros(model[u].shape) # adadelta needs one more cache
 
     # compute cost and gradient
     cg = cost_function(batch, model)