fix docstring of oneflow.pow

[doombeaker]

纠正 oneflow.pow API 文档的问题。

纠正前：

纠正后：

[github-actions]

View latest API docs preview at: https://staging.oneflow.info/docs/Oneflow-Inc/oneflow/pr/8960/

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

✔️ OneFlow resnet50 time: 128.3ms (= 12829.5ms / 100, input_shape=[16, 3, 224, 224])
PyTorch resnet50 time: 145.7ms (= 14567.6ms / 100, input_shape=[16, 3, 224, 224])
✔️ Relative speed: 1.14 (= 145.7ms / 128.3ms)

OneFlow resnet50 time: 75.5ms (= 7545.8ms / 100, input_shape=[8, 3, 224, 224])
PyTorch resnet50 time: 83.9ms (= 8394.4ms / 100, input_shape=[8, 3, 224, 224])
✔️ Relative speed: 1.11 (= 83.9ms / 75.5ms)

OneFlow resnet50 time: 48.5ms (= 9690.3ms / 200, input_shape=[4, 3, 224, 224])
PyTorch resnet50 time: 57.6ms (= 11527.9ms / 200, input_shape=[4, 3, 224, 224])
✔️ Relative speed: 1.19 (= 57.6ms / 48.5ms)

OneFlow resnet50 time: 36.3ms (= 7254.8ms / 200, input_shape=[2, 3, 224, 224])
PyTorch resnet50 time: 41.0ms (= 8194.8ms / 200, input_shape=[2, 3, 224, 224])
✔️ Relative speed: 1.13 (= 41.0ms / 36.3ms)

OneFlow resnet50 time: 28.1ms (= 5627.1ms / 200, input_shape=[1, 3, 224, 224])
PyTorch resnet50 time: 38.3ms (= 7665.4ms / 200, input_shape=[1, 3, 224, 224])
✔️ Relative speed: 1.36 (= 38.3ms / 28.1ms)

OneFlow swin dataloader time: 0.265s (= 53.022s / 200, num_workers=1)
PyTorch swin dataloader time: 0.149s (= 29.820s / 200, num_workers=1)
Relative speed: 0.562 (= 0.149s / 0.265s)

OneFlow swin dataloader time: 0.070s (= 13.939s / 200, num_workers=4)
PyTorch swin dataloader time: 0.040s (= 8.012s / 200, num_workers=4)
Relative speed: 0.575 (= 0.040s / 0.070s)

OneFlow swin dataloader time: 0.041s (= 8.134s / 200, num_workers=8)
PyTorch swin dataloader time: 0.021s (= 4.298s / 200, num_workers=8)
Relative speed: 0.528 (= 0.021s / 0.041s)

❌ OneFlow resnet50 time: 136.6ms (= 13656.9ms / 100, input_shape=[16, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 160.8ms (= 16081.8ms / 100, input_shape=[16, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.18 (= 160.8ms / 136.6ms)

OneFlow resnet50 time: 84.6ms (= 8458.7ms / 100, input_shape=[8, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 102.1ms (= 10207.8ms / 100, input_shape=[8, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.21 (= 102.1ms / 84.6ms)

OneFlow resnet50 time: 57.9ms (= 11580.2ms / 200, input_shape=[4, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 78.4ms (= 15673.6ms / 200, input_shape=[4, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.35 (= 78.4ms / 57.9ms)

OneFlow resnet50 time: 45.2ms (= 9030.9ms / 200, input_shape=[2, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 71.8ms (= 14351.3ms / 200, input_shape=[2, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.59 (= 71.8ms / 45.2ms)

OneFlow resnet50 time: 39.1ms (= 7822.6ms / 200, input_shape=[1, 3, 224, 224], ddp, world size=2)
PyTorch resnet50 time: 71.1ms (= 14214.4ms / 200, input_shape=[1, 3, 224, 224], ddp, world size=2)
✔️ Relative speed: 1.82 (= 71.1ms / 39.1ms)