Polymorph description notes that natural attacks use strength modifier for damage
An unchained rogue with a natural attack and finesse training (natural attacks.... Or claw if you aren't using feat tax rules) applies dex damage to that attack.
But if the unchained rogue is under a polymorph effect, which trumps the other? It seems like finesse training is the more specific of the two, but polymorph subschool is also a specific rule I believe. At least, I can't think of any other ability that overrides polymorph rules.
The polymorph subschool part only repeats the general rules*. Not that they aren't general rules, and class features more specific, anyway.
*) The very basic rules, i.e. the one for weapon damage rolls: "When you hit with a melee or thrown weapon, including a sling, add your Strength modifier to the damage result." CRB pg. 179 The natural attack rules only reference those rules themself, even explicitly so: "(plus your Strength modifier, as normal)" CRB pg. 182