include `pytorch-fast-transformers` as an optional module

modules/architecture/architecture_factory.py

 from .autoencoder import Autoencoder
 from .transformer import Transformer
 from .pytorch_transformer import PytorchTransformer
-from .fast_transformer import FastTransformer
-from .fast_recurrent_transformer import FastRecurrentTransformer
 
 
 def create_architecture(
@@ -61,8 +59,12 @@ def create_architecture(
     elif architecture == "pytorch":
         return PytorchTransformer(**kwargs)
     elif architecture == "fast":
+        # include `pytorch-fast-transformers` as an optional module
+        from .fast_transformer import FastTransformer
         return FastTransformer(**kwargs)
     elif architecture == "fast-recurrent":
+        # include `pytorch-fast-transformers` as an optional module
+        from .fast_recurrent_transformer import FastRecurrentTransformer
         return FastRecurrentTransformer(**kwargs)
     else:
         raise ValueError(f"ERROR: {attention}_{architecture} transformer architecture not implemented.")