Merge #1800

1800: Add more context on the behavior of the GPU function r=ToucheSir a=logankilpatrick

Following up the discussion here: #1634


Co-authored-by: Logan Kilpatrick <23kilpatrick23@gmail.com>

Author: bors[bot]

docs/src/gpu.md

@@ -49,7 +49,7 @@ m = fmap(cu, m)
 d(cu(rand(10)))
 ```
 
-As a convenience, Flux provides the `gpu` function to convert models and data to the GPU if one is available. By default, it'll do nothing, but loading `CUDA` will cause it to move data to the GPU instead.
+As a convenience, Flux provides the `gpu` function to convert models and data to the GPU if one is available. By default, it'll do nothing. So, you can safely call `gpu` on some data or model (as shown below), and the code will not error, regardless of whether the GPU is available or not. If the GPU library (CUDA.jl) loads successfully, `gpu` will move data from the CPU to the GPU. As is shown below, this will change the type of something like a regular array to a `CuArray`.
 
 ```julia
 julia> using Flux, CUDA