[Bug Fix]Fix nightly gpu test (facebookresearch#2396)

* fix nightly gpu test

* fix format

parlai/agents/transformer/modules.py

@@ -69,11 +69,14 @@ def _build_encoder(
     n_positions=1024,
     n_segments=0,
 ):
+    n_layers = (
+        opt['n_encoder_layers']
+        if opt.get('n_encoder_layers', -1) > 0
+        else opt['n_layers']
+    )
     return TransformerEncoder(
         n_heads=opt['n_heads'],
-        n_layers=(
-            opt['n_encoder_layers'] if opt['n_encoder_layers'] > 0 else opt['n_layers']
-        ),
+        n_layers=n_layers,
         embedding_size=opt['embedding_size'],
         ffn_size=opt['ffn_size'],
         vocabulary_size=len(dictionary),
@@ -96,11 +99,14 @@ def _build_encoder(
 def _build_decoder(
     opt, dictionary, embedding=None, padding_idx=None, n_positions=1024, n_segments=0
 ):
+    n_layers = (
+        opt['n_decoder_layers']
+        if opt.get('n_decoder_layers', -1) > 0
+        else opt['n_layers']
+    )
     return TransformerDecoder(
         n_heads=opt['n_heads'],
-        n_layers=(
-            opt['n_decoder_layers'] if opt['n_decoder_layers'] > 0 else opt['n_layers']
-        ),
+        n_layers=n_layers,
         embedding_size=opt['embedding_size'],
         ffn_size=opt['ffn_size'],
         vocabulary_size=len(dictionary),