fix multi machine test

[daquexian]

修复读 ONEFLOW_TEST_NODE_NUM 环境变量得到的是字符串而不是数字导致总是跳过多机测试的 bug

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 140.5ms (= 7024.6ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 128.2ms (= 6408.6ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 140.5ms / 128.2ms)

PyTorch resnet50 time: 85.0ms (= 4251.3ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 74.4ms (= 3721.9ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.14 (= 85.0ms / 74.4ms)

PyTorch resnet50 time: 58.3ms (= 2913.1ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 47.1ms (= 2354.3ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.24 (= 58.3ms / 47.1ms)

PyTorch resnet50 time: 47.2ms (= 2359.2ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 41.2ms (= 2058.2ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.15 (= 47.2ms / 41.2ms)

PyTorch resnet50 time: 43.5ms (= 2172.5ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 39.3ms (= 1966.7ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 43.5ms / 39.3ms)