PT preload_from_files ignores when no params are matching

[albertz]

There are various cases, e.g. whether we import for train, or for recog, or also just randomly initialize. Maybe it depends on the case whether it is ok to ignore when no params are matching. But I'm not sure.

E.g. for random init, we just print this when no params are matching to pattern:

print("(No relevant parameters matching.)", file=log.v3)

If there is prefix, but it does not match any parameters, this is also ignored.

I know have the case where this was a bug. E.g. consider that you have a typo in prefix.

Is there a good reason we ignore this? In the code, we e.g. could check that prefix_keys is non-empty.

I'm not sure this would break some setups now, which rely on having some dummy invalid entries which are then ignored?

(cc @NeoLegends @vieting)