[GPT2] Correct gradient checkpointing (#9308)

* correct gpt2

* fix gpt2

* fix use_cache ordering

* correct past tolerance

* fix for all cases

* style

src/transformers/models/gpt2/modeling_gpt2.py

@@ -184,9 +184,9 @@ class Attention(nn.Module):
         if head_mask is not None:
             w = w * head_mask
 
-        outputs = [torch.matmul(w, v)]
+        outputs = (torch.matmul(w, v),)
         if output_attentions:
-            outputs.append(w)
+            outputs += (w,)
         return outputs
 
     def merge_heads(self, x):
@@ -234,7 +234,7 @@ class Attention(nn.Module):
         if use_cache is True:
             present = torch.stack((key.transpose(-2, -1), value))  # transpose to have same shapes for stacking
         else:
-            present = (None,)
+            present = None
 
         attn_outputs = self._attn(query, key, value, attention_mask, head_mask, output_attentions)
         a = attn_outputs[0]
@@ -243,8 +243,7 @@ class Attention(nn.Module):
         a = self.c_proj(a)
         a = self.resid_dropout(a)
 
-        outputs = [a, present] + attn_outputs[1:]
-        return outputs  # a, present, (attentions)
+        return (a, present) + attn_outputs[1:]  # a, present, (attentions)
 
 
 class MLP(nn.Module):
@@ -321,7 +320,11 @@ class Block(nn.Module):
         # residual connection
         hidden_states = hidden_states + feed_forward_hidden_states
 
-        outputs = [hidden_states] + outputs
+        if use_cache:
+            outputs = (hidden_states,) + outputs
+        else:
+            outputs = (hidden_states,) + outputs[1:]
+
         return outputs  # hidden_states, present, (attentions, cross_attentions)
 
 
@@ -740,14 +743,14 @@ class GPT2Model(GPT2PreTrainedModel):
                     output_attentions=output_attentions,
                 )
 
-            hidden_states, present = outputs[:2]
+            hidden_states = outputs[0]
             if use_cache is True:
-                presents = presents + (present,)
+                presents = presents + (outputs[1],)
 
             if output_attentions:
-                all_self_attentions = all_self_attentions + (outputs[2],)
+                all_self_attentions = all_self_attentions + (outputs[2 if use_cache else 1],)
                 if self.config.add_cross_attention:
-                    all_cross_attentions = all_cross_attentions + (outputs[3],)
+                    all_cross_attentions = all_cross_attentions + (outputs[3 if use_cache else 2],)
 
             # Model Parallel: If it's the last layer for that device, put things on the next device
             if self.model_parallel: