Rename `RV_dims` and `initial_values` model mapping variables

[ricardoV94]

For consistency with the changes introduced in #6281, I would rename:

• model.initial_values to rvs_to_initial_values
• _RV_dims to rvs_to_dims (and loose the underscore prefix, since we don't have any model function to set those?)

Having clear consistent mapping names is very helpful for model transformations such as in pymc-devs/pymc-extras#91

Eventually we might add a helper model.replace_rv that takes care of these mappings, but this issue is just about normalizing names :)