Add masking and fast decoding to AE Transformer, remove 2 obsolete models.

PiperOrigin-RevId: 175615296

Author: Lukasz Kaiser

tensor2tensor/models/__init__.py

@@ -37,8 +37,6 @@
 from tensor2tensor.models import shake_shake
 from tensor2tensor.models import slicenet
 from tensor2tensor.models import transformer
-from tensor2tensor.models import transformer_adv
-from tensor2tensor.models import transformer_alternative
 from tensor2tensor.models import transformer_moe
 from tensor2tensor.models import transformer_revnet
 from tensor2tensor.models import transformer_sketch