sum fixes in embeddings + head, as well as new composite embedding and loss

data/transform/check_sequence_length_transform.py

modules/generative_head/composite_head_D.py

modules/generative_head/double_substitution_head.py

modules/generative_head/head_factory.py

modules/generative_head/substitution_head.py

modules/token_embedding/composite_embedding_D.py

modules/token_embedding/double_substitution_embedding.py

modules/token_embedding/embedding_factory.py

modules/token_embedding/substitution_embedding.py

utils/loss/loss_factory.py