[Task] Transformers4Rec Serving using Merlin Schema

[EvenOldridge]

• [ ] Remove dependency of Merlin stdlib
• [ ] Saving Merlin input schema when saving the model

[viswa-nvidia]

Moving this to T4R repo.

[sararb]

I updated the task description, as what is needed for T4Rec serving is saving input and output merlin-core schemas. This fix #507 implements input/output_schema methods in T4Rec that build the schemas using the merlin-core Schema class.

Removing the merlin-standard dependency is a breaking change that will require a larger refectory of multiple T4Rec blocks and is outside of the scope of this ticket.