Add/Fix the doc strings for PyT serving support for T4R models in TorchScript

Reviewed [API documentation](https://nvidia-merlin.github.io/Transformers4Rec/main/api/modules.html) (generated by docstrings in code) for Transformers4Rec/transformers4rec codebase. Details are available here: https://docs.google.com/document/d/1GzM1q9Pc6amecRspbWKG0PSXbU-fCy13hSOC6UMnWWU/edit