[INF] Merlin commons - Schemas

[viswa-nvidia]

• Merlin schema file formats - Create our own schema file format outside of the TF format to allow for easier storing of metadata #824
• Model schemas - Make sure all libraries output a model schema (Implicit, LightFM, XGBoost don't output schemas) #825
• Dtype Mappings - Clean up mappings and remove translations happening in the code, replacing with schema based equivalent. #826
• T4R has its own schema - make T4R consistent with Merlin schema]([FEA] Update Transformers4Rec to use merlin core schema  Transformers4Rec#566)
• Migrate T4Rec tags to merlin.schema.Tags #844
• Allow Schema class from core to work with input-blocks Transformers4Rec#635
• Allow Schema class from core to work with Trainer #852