[BE] Get rid of `six` in caffe2 code (#93956)
Mostly `s/string_types/str/` `s/binary_types/bytes/` and `s/text_types/str/`
Also `y.extend([str(x) for x in foo])`->`y.extend(map(str, foo))`
As Python-2 is long dead
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93956
Approved by: https://github.com/albanD, https://github.com/Skylion007
