support skip atten in export (#16104)

Summary:

Support export for llama model variants with attention layer skipping. We only need to specify the attention skip patterns in config.json in layer_type. E.g.,

"layer_types": [
    "full_attention",
    "full_attention",
    "full_attention",
    "skip_attention",
    "skip_attention",
    "skip_attention"
  ]

Differential Revision: D88399533

Author: Hanxian97

examples/models/llama/llama_transformer.py

@@ -272,6 +272,10 @@ def construct_transformer(model_args: ModelArgs) -> Transformer:
                     norm_eps=model_args.norm_eps,
                 )
             )
+        elif model_args.layer_types and model_args.layer_types[layer_id] == "skip_attention":
+            attention = AttentionSkip()
+            transformer_block = TransformerBlock(model_args, attention)
+            layers.append(transformer_block)
         else:
             attention = cls(
                 model_args, layer_id, rope, **model_args.attention_kwargs