[Pooling] Stride default size issue

[Donghak PARK]

Issue

• If the user does not enter the stride value when creating the pooling2d layer in the current NNTrainer, 1 is set to the default value.
• in other framework (tensorflow, pytorch) they set default value as kernel size
• default stride size = kernel size is reasonable in most case -> user use pooling for reduce size

NYI) below is other framework's MaxPool2D's args

tensorflow : https://www.tensorflow.org/api_docs/python/tf/keras/layers/MaxPool2D#args
pytorch : https://pytorch.org/docs/stable/generated/torch.nn.MaxPool2d.html

[taos-ci]

cibot: Thank you for posting issue #2144. The person in charge will reply soon.