process auto output variable

[lucemia]

由於 split, asplit 已經被處理了，
只有 select, aselect 會需要這種處理

{
  "__class__": "scripts.manual.schema.FFMpegFilterManuallyDefined",
  "formula_typings_input": null,
  "formula_typings_output": "[StreamType.audio] * int(outputs)",
  "name": "asplit"
}