_get_f_placeholder_args: set ForceValueArgTag (#304)

* _get_f_placeholder_args: set ForceValueArgTag

* Update requirements.txt

* skip scalar arg handling in _args_to_device_buffers

* Revert changes to requirements.txt

* add a simple test

* Fix test

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Co-authored-by: Andreas Klöckner <inform@tiker.net>