Improve GPU update tasks assignment consistency

Improve GPU update tasks assignment consistency

GPU update task assignment was not consistent with the assumptions and
supported features of the 2020 release and did not implement the correct
checks and fallback in cases where GPU update was decided to not be
supported. Specifically, this change makes sure that when separate PME
ranks are used, without direct GPU communication for PP-PME, GPU update
falls back to the CPU.

Fixes #3354

Change-Id: I7c9dd67cd8cf61f0201b626b8b7674917e3365a5