Fixed bug for 16a4w ptq

[rohansjoshi]

Summary: Currently running the script executorch/examples/models/llama/export_llama.py with the flag --ptq 16a4w, it does 16a16w quantization; this diff fixes this. This may be related to some GitHub issues

Differential Revision: D77671468

[pytorch-bot]

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This pull request was exported from Phabricator. Differential Revision: D77671468

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[cccclai]

Thanks for the fix! @haowhsu-quic @winskuo-quic @chunit-quic fyi for the fix