Add clip_grad_norm

[zjlablichenyang]

clip_grad_norm_ docstring

clip_grad_norm_ doctest

[github-actions]

CI failed, removing label automerge

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 140.6ms (= 7028.0ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 126.3ms (= 6313.9ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.11 (= 140.6ms / 126.3ms)

PyTorch resnet50 time: 84.9ms (= 4243.5ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 73.7ms (= 3683.5ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.15 (= 84.9ms / 73.7ms)

PyTorch resnet50 time: 63.9ms (= 3193.3ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 48.1ms (= 2405.2ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.33 (= 63.9ms / 48.1ms)

PyTorch resnet50 time: 49.8ms (= 2492.4ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 43.7ms (= 2182.5ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.14 (= 49.8ms / 43.7ms)

PyTorch resnet50 time: 44.9ms (= 2246.7ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 41.3ms (= 2067.3ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 1.09 (= 44.9ms / 41.3ms)