Merge #1607

1607: document Join and Split error r=DhairyaLGandhi a=magicly

```julia
(m::Join)(xs::Tuple) = m.combine(map((f, x) -> f(x), m.paths, xs))
```
will get a `Vector` of `Tuple` of `Vector`.  Should use `mapreduce` as `Flux.Parallel` or just use `(m::Join)(xs::Tuple) = m.combine(map((f, x) -> f(x), m.paths, xs)...)`.

`CustomSplit` should be `Split`.




Co-authored-by: magicly <magicly007@gmail.com>

Author: bors[bot]

docs/src/models/advanced.md

@@ -104,7 +104,7 @@ Flux.@functor Join
 
 Finally, we define the forward pass. For `Join`, this means applying each `path` in `paths` to each input array, then using `combine` to merge the results.
 ```julia
-(m::Join)(xs::Tuple) = m.combine(map((f, x) -> f(x), m.paths, xs))
+(m::Join)(xs::Tuple) = m.combine(map((f, x) -> f(x), m.paths, xs)...)
 (m::Join)(xs...) = m(xs)
 ```
 
@@ -172,7 +172,7 @@ Now we can test to see that our `Split` does indeed produce multiple outputs.
 ```julia
 model = Chain(
               Dense(10, 5),
-              CustomSplit(Dense(5, 1),Dense(5, 3),Dense(5, 2))
+              Split(Dense(5, 1),Dense(5, 3),Dense(5, 2))
              ) |> gpu
 
 model(gpu(rand(10)))