[Task] Add/Fix the doc strings for TF based session based API documentation

[bbozkaya]

Related: NVIDIA-Merlin/Merlin#433
Note: For 23.05 we will only focus on findings related to session based in the API review doc posted by Burcin below

[bbozkaya]

Completed review of API documentation (generated by docstrings in code) for Merlin Models codebase. Details are available here: https://docs.google.com/document/d/1v4rk2-pW34swfbDWz_d41ADlXjUcT9Yoc_xO3YgN02U/edit

[viswa-nvidia]

@nv-alaiacano , please merge the changes to 23.05 release branch and close this ticket.