Unused contents of the offset_provider dict generate errors with gtfn backend

[egparedes]

Passing unused NeighborTableOffsetProvider items in the offset provider of a cartesian program in gt4py.next (either explicitly set as cartesian or detected automatically) results in errors with the gtfn backend. Moreover, it also seems that passing an offset provider with unused cartesian Dimensions items to unstructured programs also generate an error (although a different one).

This behavior is weird and it would more natural to ignore the content of offset providers which are not used in the program.

This issue was originally found and reported by @nfarabullini and @halungge in the icon4py project (e.g. https://github.com/C2SM/icon4py/blob/db0dbd65ba5d449b483fdf98fd47acdcb1ddf11e/model/common/src/icon4py/model/common/metrics/metric_fields.py#L228)