Incorrect results when nesting ReverseDiff inside ForwardDiff

I'm not sure if this is even supported, but I think it should throw an error instead of returning zero gradients

The first example is an old bug (#45 ), the second shows you can't nest ReverseDiff inside ForwardDiff, the third shows you can the other way around.

julia> let
           D(f, x) = ReverseDiff.gradient(x->f(x[1]), [x])[1]
           u1 = D(x -> x * D(y -> x * y, 3), 5) # 3
           u2 = D(x -> x * D(y -> y * x, 3), 5) # 5
           @show u1, u2
       end
(u1, u2) = (3, 5)
(3, 5)

julia> let
           Dr(f, x) = ReverseDiff.gradient(x->f(x[1]), [x])[1]
           Df(f, x) = ForwardDiff.gradient(x->f(x[1]), [x])[1]
           u1 = Df(x -> x * Dr(y -> x * y, 3), 5) 
           u2 = Df(x -> x * Dr(y -> y * x, 3), 5) 
           @show u1, u2
       end
(u1, u2) = (0, 0)
(0, 0)

julia> let
           Dr(f, x) = ReverseDiff.gradient(x->f(x[1]), [x])[1]
           Df(f, x) = ForwardDiff.gradient(x->f(x[1]), [x])[1]
           u1 = Dr(x -> x * Df(y -> x * y, 3), 5) 
           u2 = Dr(x -> x * Df(y -> y * x, 3), 5) 
           @show u1, u2
       end
(u1, u2) = (10, 10)
(10, 10)

(test3) pkg> status
Status `~/Sandboxes/Julia-Misc/test3/Project.toml`
  [f6369f11] ForwardDiff v0.10.16 `~/.julia/dev/ForwardDiff`
  [37e2e3b7] ReverseDiff v1.5.0 `~/.julia/dev/ReverseDiff`

julia> versioninfo()
Julia Version 1.5.3
Commit 788b2c77c1 (2020-11-09 13:37 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: AMD Ryzen 7 3700X 8-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-9.0.1 (ORCJIT, znver2)

Reproduce the nested calls using the ReverseDiff.gradient and ForwardDiff.gradient entry points shown in the issue with the listed Julia and package versions. Determine the intended behavior for ReverseDiff inside ForwardDiff, then add coverage for the reported zero-gradient case and verify that the resulting behavior is explicit and consistent.