added identity warp function and change np.argmin to np.nanargmin in optimize_restarts

GPy/core/model.py

@@ -104,7 +104,7 @@ class Model(Parameterized):
                     raise e
 
         if len(self.optimization_runs):
-            i = np.argmin([o.f_opt for o in self.optimization_runs])
+            i = np.nanargmin([o.f_opt for o in self.optimization_runs])
             self.optimizer_array = self.optimization_runs[i].x_opt
         else:
             self.optimizer_array = initial_parameters

GPy/util/warping_functions.py

@@ -293,3 +293,25 @@ class TanhWarpingFunction_d(WarpingFunction):
         names = sum([['warp_tanh_%s_t%i' % (variables[n],q) for n in range(3)] for q in range(self.n_terms)],[])
         names.append('warp_tanh_d')
         return names
+
+
+class IdentityFunction(WarpingFunction):
+    """
+    Identity warping function. This is for testing and sanity check purposes
+    and should not be used in practice.
+    """
+    def __init__(self):
+        self.num_parameters = 0
+        super(IdentityFunction, self).__init__(name='identity')
+        
+    def f(self, y):
+        return y
+
+    def fgrad_y(self, y):
+        return 1.0
+
+    def fgrad_y_psi(self,y):
+        return 1.0
+
+    def f_inv(self,z):
+        return z