[Task] Enable pickle serialzation of Models with Merlin Schema Saved alongside

[oliverholworthy]

Description

Enable pickle serialzation of Models with Merlin Schema Saved alongside.

Part of #669

Additional context

Noticed on this PR NVIDIA-Merlin/Merlin#770

We currently get a FileNotFoundError when tryting to pickle a model.

This is because keras uses the tf.io in-memory filesystem which I think is a tensorflow io feature (I've not seen ram:// used outside of this context at least) when trying to pickle a model. And to use this RAM filesystem, you can't use regular python file io functionality it seems. We'll need to use tf.io functions to write things into this in-memory filesystem.

This looks like it's been replaced in TF 2.11 by a regular regular temporary directory on disk. However, since we currently aim to support versions back to TF 2.8, will need to handle both cases now.