[GPTQ] Fix actorder resolution, add sentinel

[kylesayrs]

Purpose

• Fix false assumption that actorder field is of enum type
  • Despite the fact that actorder passes through a field_validator, QuantizationArgs has the use_enum_values configuration set, meaning that enum values are converted to strings.
  • This was done in relation to this fix
• Remove conflict with recipes which manually specify activation ordering by using a sentinel value

Follow ups

• [GPTQ] Change actorder default to "static" #1425

Testing

• Ran llama3 example with manually specified actorder=group

[github-actions]

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[dsikka]

Quick question before approving