support skip atten in export

[jackzhxng]

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

[pytorch-bot]

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[lucylq]

Is 'skip_attention' standard?

[Hanxian97]

yes it is a standard name if we want to call https://github.com/pytorch/executorch/blob/main/examples/models/llama/attention.py#L525