remove nccl True in unit test

Author: QiJune

python/paddle/fluid/tests/book_memory_optimization/test_memopt_fit_a_line.py

@@ -31,10 +31,10 @@
 place = fluid.CPUPlace()
 if fluid.core.is_compiled_with_cuda():
     device_type = 'CUDA'
-    use_nccl = True
+    use_nccl = False
     place = fluid.CUDAPlace(0)
 
-places = fluid.layers.get_places(device_count=2, device_type=device_type)
+places = fluid.layers.get_places(device_count=0, device_type=device_type)
 pd = fluid.layers.ParallelDo(places, use_nccl=use_nccl)
 with pd.do():
     x_ = pd.read_input(x)