Rename cpu to async cpu

[lixinqi]

将CpuStreamType重命名为AsyncCpuStreamType，与AsyncCudaStreamType对齐。

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 139.3ms (= 6963.0ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 126.6ms (= 6332.3ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 139.3ms / 126.6ms)

PyTorch resnet50 time: 83.7ms (= 4183.8ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 76.8ms (= 3838.9ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.09 (= 83.7ms / 76.8ms)

PyTorch resnet50 time: 58.0ms (= 2900.2ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 48.6ms (= 2428.7ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.19 (= 58.0ms / 48.6ms)

PyTorch resnet50 time: 48.7ms (= 2435.6ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 41.6ms (= 2078.3ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.17 (= 48.7ms / 41.6ms)

PyTorch resnet50 time: 45.1ms (= 2253.8ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 43.2ms (= 2158.4ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 1.04 (= 45.1ms / 43.2ms)