add docstring for gradient

[johnnychen94]

partially address #53

[johnnychen94]

I don't know why this is disabled. Is it possible to return nothing for it? Otherwise it can very easily be a root of potential bugs.

[GiggleLiu]

NiLang assumes gradient type == value type. Integers can be differentiated manually by wrapping GVar. Also, I assume in most cases, people do not want to differentiate them.
Wondering what are the potential bug?

[johnnychen94]

It is not very easy to detect and understand that NiLang.AD.gradient(Val(1), i_sum, (0, X))[2] is zeros (maybe throw a warning for this?)

Another example comes from the Zygote example, and it's quite confusing from the error stack:

julia> norm2'([1,2,3,4])
4-element Vector{Int64}:
 2
 4
 6
 8

julia> Zygote.@adjoint function norm2(x::AbstractArray{T}) where T
           out = norm2(x)
           out, δy -> (grad((~r_norm2)(GVar(out, δy), GVar(x))[2]),)
       end

julia> norm2'([1,2,3,4])
ERROR: MethodError: no method matching (::Inv{typeof(r_norm2)})(::GVar{Int64, Int64}, ::Vector{Int64})
Closest candidates are:
  (::Inv{typeof(r_norm2)})(::T, ::AbstractArray{T, N} where N) where T at REPL[8]:2
Stacktrace:
 [1] (::var"#10#11"{Vector{Int64}, Int64})(δy::Int64)
   @ Main ./REPL[11]:3
 [2] (::var"#138#back#12"{var"#10#11"{Vector{Int64}, Int64}})(Δ::Int64)
   @ Main ~/.julia/packages/ZygoteRules/OjfTt/src/adjoint.jl:59
 [3] (::Zygote.var"#41#42"{var"#138#back#12"{var"#10#11"{Vector{Int64}, Int64}}})(Δ::Int64)
   @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface.jl:41

[GiggleLiu]

Here is how you can circumvent this issue:

julia> Zygote.@adjoint function norm2(x::AbstractArray{T}) where T
               out = norm2(x)
               out, δy -> (grad((~r_norm2)(GVar(out, δy), GVar(x, zero(x)))[2]),)
          end

julia> norm2'([1,2,3,4])
4-element Vector{Int64}:
 2
 4
 6
 8

Does this make sense?