[Bug] Numpy backend doesn't handle non-literal accessor

[FlorianDeconinck]

Tracks: GridTools/gt4py#1578

Parent: GEOS-ESM/SMT-Nebulae#36

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• Fixed [cartesian] Numpy backend lacks non-literal accessor for data dimensions GridTools/gt4py#1578

[FlorianDeconinck]

4D POC in https://github.com/FlorianDeconinck/gt4py/tree/cartesian/fix/numpy_variable_ddim_access