UnitContinuousDistribution does not init the default transform

[lucianopaz]

I was looking a bit into the transforms in pymc4 after a discussion about transforms in pymc3 and I noticed that the UnitContinuousDistribution does not implement its default transform (we do this for the PositiveContinuousDistribution).

This should be simple to solve. We should just copy what we did for the PositiveContinuousDistribution but use the Sigmoid bijector instead of the Log by default.