set CMAKE_BUILD_TYPE to Release if undefined

[daquexian]

修复 #5656 带来的问题，在不设置 CMAKE_BUILD_TYPE 时将它设置成 Release

[github-actions]

Speed stats:

GPU Name: GeForce GTX 1080 

PyTorch resnet50 time: 141.4ms (= 7068.7ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 128.2ms (= 6410.6ms / 50, input_shape=[16, 3, 224, 224], backward is enabled)
Relative speed: 1.10 (= 141.4ms / 128.2ms)

PyTorch resnet50 time: 84.5ms (= 4226.0ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 74.5ms (= 3724.2ms / 50, input_shape=[8, 3, 224, 224], backward is enabled)
Relative speed: 1.13 (= 84.5ms / 74.5ms)

PyTorch resnet50 time: 60.1ms (= 3006.7ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 47.4ms (= 2368.6ms / 50, input_shape=[4, 3, 224, 224], backward is enabled)
Relative speed: 1.27 (= 60.1ms / 47.4ms)

PyTorch resnet50 time: 49.2ms (= 2461.3ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 40.1ms (= 2005.4ms / 50, input_shape=[2, 3, 224, 224], backward is enabled)
Relative speed: 1.23 (= 49.2ms / 40.1ms)

PyTorch resnet50 time: 38.0ms (= 1900.9ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
OneFlow resnet50 time: 39.1ms (= 1956.2ms / 50, input_shape=[1, 3, 224, 224], backward is enabled)
Relative speed: 0.97 (= 38.0ms / 39.1ms)