Use stateless random ops

[albertz]

This makes all logic regarding randomness much more explicit.

The current sources of randomness are:

• dropout
• rand_int
• Param init, although the technical details are still being discussed (How to handle parameter initialization #92)

Maybe we can reimplement dropout here, or make this optionally using stateless random ops on RETURNN side.

RandIntLayer on RETURNN side should also have an option for using the stateless random op.

For param init, we probably need all the other common random ops anyway, like random_uniform, random_normal, random_truncated_normal. And then we directly should use the stateless variants.

[albertz]

We now have the generic RandomLayer (via rwth-i6/returnn#911) which supports stateless random ops, or rather explicit handling of state.

[albertz]

Should dropout still use the functional API or become a Module? Should we always make the state var for RandomLayer (nn.random) explicit or is this maybe something where we allow an exception?

Currently I tend to keep dropout as a function...

[albertz]

I think nothing really needs to be done here for now.