fix var bug

[liufengwei0103]

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Speed stats:

GPU Name: GeForce GTX 1080 

✔️ OneFlow resnet50 time: 128.5ms (= 12850.9ms / 100, input_shape=[16, 3, 224, 224])
PyTorch resnet50 time: 141.6ms (= 14159.5ms / 100, input_shape=[16, 3, 224, 224])
✔️ Relative speed: 1.10 (= 141.6ms / 128.5ms)

✔️ OneFlow resnet50 time: 78.0ms (= 7795.4ms / 100, input_shape=[8, 3, 224, 224])
PyTorch resnet50 time: 85.4ms (= 8539.4ms / 100, input_shape=[8, 3, 224, 224])
✔️ Relative speed: 1.10 (= 85.4ms / 78.0ms)

OneFlow resnet50 time: 51.2ms (= 10236.0ms / 200, input_shape=[4, 3, 224, 224])
PyTorch resnet50 time: 58.6ms (= 11711.8ms / 200, input_shape=[4, 3, 224, 224])
✔️ Relative speed: 1.14 (= 58.6ms / 51.2ms)

OneFlow resnet50 time: 41.8ms (= 8369.4ms / 200, input_shape=[2, 3, 224, 224])
PyTorch resnet50 time: 43.7ms (= 8745.6ms / 200, input_shape=[2, 3, 224, 224])
✔️ Relative speed: 1.04 (= 43.7ms / 41.8ms)

OneFlow resnet50 time: 38.8ms (= 7756.8ms / 200, input_shape=[1, 3, 224, 224])
PyTorch resnet50 time: 38.2ms (= 7631.4ms / 200, input_shape=[1, 3, 224, 224])
✔️ Relative speed: 0.98 (= 38.2ms / 38.8ms)

✔️ OneFlow resnet50 time: 141.6ms (= 14155.3ms / 100, input_shape=[16, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 162.1ms (= 16214.3ms / 100, input_shape=[16, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.15 (= 162.1ms / 141.6ms)

OneFlow resnet50 time: 89.4ms (= 8935.6ms / 100, input_shape=[8, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 101.5ms (= 10152.1ms / 100, input_shape=[8, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.14 (= 101.5ms / 89.4ms)

OneFlow resnet50 time: 62.3ms (= 12466.0ms / 200, input_shape=[4, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 73.4ms (= 14680.9ms / 200, input_shape=[4, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.18 (= 73.4ms / 62.3ms)

OneFlow resnet50 time: 51.7ms (= 10348.7ms / 200, input_shape=[2, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 68.4ms (= 13686.1ms / 200, input_shape=[2, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.32 (= 68.4ms / 51.7ms)

OneFlow resnet50 time: 49.7ms (= 9940.7ms / 200, input_shape=[1, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 59.1ms (= 11812.4ms / 200, input_shape=[1, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.19 (= 59.1ms / 49.7ms)