[typing] Make arguments to __getitem__/__setitem__/ etc. positional only.

[randolf-scholz]

🚀 The feature, motivation and pitch

A strict type checker will complain when overwriting __getitem__ with a different argument:

from torch.utils.data import Dataset

class MyDatasets(Dataset):
    def __getitem__(self, key, /) -> int:
        return 0

Gives: Parameter 2 mismatch: base parameter "index" is keyword parameter, override parameter is position-only when checking with pyright.

Note: most type-checkers support the white lie that if the parent class defines __getitem__(self, index), then a subclass could do __getitem__(self, key), despite this not being type save. This could change in the future as more libraries adopt positional only parameters.

Additional context

Such changes are backward-compatibility braking if a caller tries to execute something like A.__getitem__(self=self, index=index).

cc @ezyang @malfet @xuzhao9 @gramster

[ezyang]

Sgtm