Align where op with torch

[BBuf]

支持where只传condition，和oneflow.nonzeros(condition, as_tuple=True)等价。和Pytorch对齐用法。

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 139.9ms (= 6994.9ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 128.5ms (= 6422.7ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.09 (= 139.9ms / 128.5ms)

PyTorch resnet50 time: 83.5ms (= 4177.1ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 74.5ms (= 3722.8ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.12 (= 83.5ms / 74.5ms)

PyTorch resnet50 time: 58.2ms (= 2907.7ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 48.5ms (= 2423.6ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.20 (= 58.2ms / 48.5ms)

PyTorch resnet50 time: 48.7ms (= 2436.4ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 40.2ms (= 2010.9ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.21 (= 48.7ms / 40.2ms)

PyTorch resnet50 time: 44.0ms (= 2199.9ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 39.9ms (= 1994.0ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 44.0ms / 39.9ms)