Switch to C++20

[kwasd]

possible fix for #4890

[kwasd]

Any possible problems on upstreaming further changes since this diverges with upstream?

[kwasd]

@etiotto

Can we simply change the std level to C++20 instead of changing the code ?

For me it seems like this is too early to switch.
As of now, PyTorch use C++17: https://github.com/pytorch/pytorch/blob/9903ca4f70bdc1653016256f5b4fd74fdfc609f8/CMakeLists.txt#L45

It seems like today #4889 is a better fix.

By Anatoly
#2833 (comment):

Do you know if upstream is opened to changing to C++20 compliant?

I don't know, it seems to me that there is a connection with PyTorch, which still uses c++17 standard. I suppose that as soon as it updates the standard, Triton will do the same.