Random Numbers & Reactant

[avik-pal]

Note that any layers using random numbers -- notably Dropout and VariationalHiddenDropout will give un-expected results till EnzymeAD/Reactant.jl#356

[avik-pal]

With EnzymeAD/Reactant.jl#363 close to landing. We need to do the following:

• Add tests for Dropout and VariationalHiddenDropout layers
• overload default_device_rng to return TracedRNG