Add policy mix option to value-only mode.

[Tilps]

Bit of a hack. If we decide this is useful I should also add it to value_tournament, clean it up so it does literally nothing when mix is 0.

Running a tune with T82 network - looks like optimum is to have very low policy temperature and a moderate policy mix.
Tune probably doesn't have right range for PST - but current minima (after 6000 games) is 0.325 (policy mix) and 0.26 (policy temperature). Elo gain vs pure value mode is estimated as ~40 elo.
As the policy temperature goes down, so does the optimal policy mix. At policy temperature 1 the optimal policy mix seemed closer to 0.5. (Which was also probably >30 elo)

[Tilps]

New optima at 9000 games - {'PolicyMix': 0.14689769678592882, 'PolicyTemperature': 0.2}

[Tilps]

15000 games {'PolicyMix': 0.4831467071587293, 'PolicyTemperature': 0.2} (Elo estimate is at 49)

seems like I'll need to do another run expanding policy temperature even further...

[Tilps]

18000 games - {'PolicyMix': 0.5291014358428309, 'PolicyTemperature': 0.2} (elo estimate 47)

Going to do the restart with double number of rounds overnight.

[Tilps]

restarting with wider policy temperature (and removing the clearly bad negative policy mix values) oddly decided to go find a minima with a completely different policy temperature..
{'PolicyMix': 0.4585496867191581, 'PolicyTemperature': 1.222335248497085} (elo estimate of 45 after 36000 tuning games)

so seems like the theory that policy mix reduces as temperature reduces is invalid... Policy temperature just seems to have a relatively small effect maybe...

(There was an early minima at temperature 0.05 with PolicyMix of 0.35 - but it never stabilized there seems rare after the first 18000 tuning games.)