fix prod with tuple arg

[CarloLucibello]

This fixes the following problem caused by a relaxation of the signature in #112

julia> gradient(x -> prod((1,2,3)), 1)
ERROR: MethodError: no method matching prod(::Tuple{Int64,Int64,Int64}; dims=Colon())
Closest candidates are:
  prod(::Tuple{Any,Vararg{Any,N} where N}) at tuple.jl:385 got unsupported keyword argument "dims"
  prod(::Any) at reduce.jl:448 got unsupported keyword argument "dims"
  prod(::Any, ::StaticArrays.StaticArray{#s160,T,N} where N where #s160<:Tuple; dims) where T at /home/carlo/.julia/packages/StaticArrays/1g9bq/src/mapreduce.jl:234
  ...
Stacktrace:
 [1] #adjoint#3920 at /home/carlo/.julia/packages/Zygote/XCgv1/src/lib/array.jl:220 [inlined]
 [2] adjoint at ./none:0 [inlined]
 [3] _pullback at /home/carlo/.julia/packages/ZygoteRules/6nssF/src/adjoint.jl:47 [inlined]
 [4] #17 at ./REPL[18]:1 [inlined]
 [5] _pullback(::Zygote.Context, ::var"#17#18", ::Int64) at /home/carlo/.julia/packages/Zygote/XCgv1/src/compiler/interface2.jl:?
 [6] _pullback(::Function, ::Int64) at /home/carlo/.julia/packages/Zygote/XCgv1/src/compiler/interface.jl:31
 [7] pullback(::Function, ::Int64) at /home/carlo/.julia/packages/Zygote/XCgv1/src/compiler/interface.jl:37
 [8] gradient(::Function, ::Int64, ::Vararg{Int64,N} where N) at /home/carlo/.julia/packages/Zygote/XCgv1/src/compiler/interface.jl:46
 [9] top-level scope at REPL[18]:1

[DhairyaLGandhi]

The test should check for a valid gradient result

Suggested change

	@test gradient(x -> prod((1,2,3)), 1)[1] === nothing
	x = (1,2,3)
	@test gradient(x -> prod(x), x)[1] == (6,3,2)

[CarloLucibello]

bors r+

[bors]

Build succeeded

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