[shardformer] fix GPT2DoubleHeadsModel (#4703)

colossalai/shardformer/modeling/gpt2.py

@@ -94,9 +94,9 @@ class GPT2PipelineForwards:
             if hidden_states is None:
                 raise ValueError("hidden_states shouldn't be None for stages other than the first stage.")
             input_shape = hidden_states.size()[:-1]
-            batch_size = input_shape[0]
             device = hidden_states.device
             hidden_states = hidden_states.view((-1,) + hidden_states.shape[-2:])
+            batch_size = hidden_states.shape[0]
 
         # GPT2Attention mask.
         if attention_mask is not None: