turn off reticulate's autoconfig when initialising python

[goldingn]

in check_tf_version()

[goldingn]

I think check_tf_version() is always the first place python could be initialised in a greta session (it's called in the creation of all greta arrays, and you can't invoke TF without creating a greta array). So this should cover all bases.

The only possible exception I can think of is if a user saves some greta arrays in one session, then starts a new session and then does things with them that call python (creates a model, does MCMC, etc.). If that is the case, we will want to run check_tf_version() in those situations that call python too. We'd want that anyway as we'd want to check the versions are correct.

The heavy-handed alternative would be to set this environment variable for the whole R session, whenever greta is loaded. But that seems icky.

[njtierney]

Looks good to me, I had tried out a version where I checked for an option that we could set, like, greta.python_autoconfig, but I think this on.exit is safer as it reverts back to whatever state the user had previously set.

So it sounds like the potential issue you mentioned there could be resolved by ensuring check_tf_version() is called in other key modelling steps, like mcmc, or model ?