Need help understanding the role of the 'upfirdn' after the transpose_conv

[NoAchache]

Hello, I don't really get the point of the upfirdn (apart from reducing the dim of the height and width of x by one)?
How is the resample kernel chosen? ([1,3,3,1] in the code)
Why is the filter 4x4 (In the doc of the upfirdn of scipy, it says the filter should be 1-D https://docs.scipy.org/doc/scipy/reference/generated/scipy.signal.upfirdn.html )?

[moono]

I think it's better asked on official_repo.
Those parameters were chosen by the author of original code. I just ported (wrapped) as tensorflow 2.