Exclude test_visual_encoder_trains from GPU test (#5367)

ml-agents/mlagents/trainers/tests/torch/test_encoders.py

         (FullyConnectedVisualEncoder, 36),
     ],
 )
+@pytest.mark.check_environment_trains
 def test_visual_encoder_trains(vis_class, size):
     torch.manual_seed(0)
     image_size = (size, size, 1)