Add type-stability checks via DispatchDoctor.jl

[avik-pal]

1. We can't apply it to the entire Lux module. Some of the code exists for debugging and is intentionally type unstable.
2. I like the default of erroring, but that will be too breaking. Maybe we can consider that for v0.6. We will warn if the code is type unstable for now.

Repository: https://github.com/MilesCranmer/DispatchDoctor.jl

[avik-pal]

We can do it at multiple levels:

1. apply: this is the most aggressive one Check what happens if we enable Dispatch Checks at apply LuxCore.jl#28
2. In the LuxLib _impl functions: It makes sense to guarantee these ones to be type stable feat: using dispatch doctor for return type stability guarantees LuxLib.jl#79
3. In Lux applychain, applyparallel, ... functions which get hit pretty commonly.