Add dims for GPs

[ferrine]

Currently, GPs do not fully support dims as kwargs

https://github.com/pymc-devs/pymc-experimental/blob/main/pymc_experimental/gp/latent_approx.py#L34

A consistent implementation should go in line with

https://github.com/pymc-devs/pymc/blob/main/pymc/gp/gp.py#L135

https://github.com/pymc-devs/pymc/blob/main/pymc/gp/gp.py#L282

CC @bwengals