allow hidden dimension when calling FFlux routines (#4162) #6139

Job	Run time
HIP ROCm Flang C++17 [tests]	42m 50s
HIP EB [configure 2D]	2m 5s
HIP EB 3D GMake	17m 59s
HIP ROCm [email protected] C++17 [tests-hipcc]	15m 15s
	1h 18m 9s

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