Transformer decoder (#445)

Summary:
Pull Request resolved: #445

Add class for stack of transformer decoder layers

Reviewed By: rck-meta, ankitade

Differential Revision: D47891813

fbshipit-source-id: d58ebd5673247315cdaf18a2583f87d6e78fa274

torchmultimodal/modules/layers/transformer.py

@@ -7,7 +7,7 @@
 # Code for some of the transformers components in this file are initialized
 # from their counterparts in Hugging Face Transformers library.
 
-from typing import List, NamedTuple, Optional
+from typing import List, NamedTuple, Optional, Tuple
 
 from torch import Tensor
 
@@ -18,3 +18,4 @@ class TransformerOutput(NamedTuple):
  hidden_states: Optional[List[Tensor]] = None
  attentions: Optional[List[Tensor]] = None
  image_labels: Optional[Tensor] = None
+ current_key_values: Optional[List[Tuple[Tensor, Tensor]]] = None