[FEA] Add a warning, if embedding tables are not used in merlin models

[bschifferer]

🚀 Feature request

As a user, I can provide custom configs to the embedding_initalizer. However, the input data schema does not tag the column names as ITEM / USER features, then the model will not create embeddings and the configurations are not used.

We should throw a warning, if the configs are not used

    embeddings_initializers={ 
                             "emb1": mm.TensorInitializer(emb1), #,trainable=False
                             "emb2": mm.TensorInitializer(emb2)#,trainable=False
    }

[rnyak]

@bschifferer can you create a PR for that? can I assign this to you?