Fix layers reference

ml-agents/mlagents/trainers/torch/encoders.py

             kernel_init=Initialization.KaimingHeNormal,
             kernel_gain=1.0,
         )
-        self.sequential = nn.Sequential(*self.layers)
+        self.sequential = nn.Sequential(*layers)
 
     def forward(self, visual_obs):
         batch_size = visual_obs.shape[0]