[Cleanup] Split out transformer components: 10.1/N

Created by: suchenzang

Cleanup part... 10? Changes here include:

• removed set_beam_size and reorder_incremental_state_scripting methods in incremental_decoder.py
• moved TransformerEncoder and Embedding (moved to modules/) to separate files, renamed transformer.py -> transformer_decoder.py
• deleted unused model_utils.py file
• moved ffn and transformer encoder layer to separate files

Used a 125m baseline as a test run to confirm parity.