Restruct activation grad op

[MARD1NO]

整理激活函数grad functional，统一到一个文件下 activation.cpp

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 140.4ms (= 7021.2ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 126.7ms (= 6337.1ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.11 (= 140.4ms / 126.7ms)

PyTorch resnet50 time: 83.5ms (= 4175.2ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 74.1ms (= 3705.2ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.13 (= 83.5ms / 74.1ms)

PyTorch resnet50 time: 58.1ms (= 2906.1ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 47.9ms (= 2392.6ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.21 (= 58.1ms / 47.9ms)

PyTorch resnet50 time: 50.0ms (= 2499.1ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 42.8ms (= 2141.5ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.17 (= 50.0ms / 42.8ms)

PyTorch resnet50 time: 44.4ms (= 2219.3ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 43.7ms (= 2183.1ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 1.02 (= 44.4ms / 43.7ms)