fix: CUDA package optional for FluxMPIExt

[askorupka]

Hotfix for #2464 the issue reported by @kishore-nori (#2464 (comment))

[kishore-nori]

Thank you very much @askorupka

[askorupka]

You're welcome and thanks for reporting the bug @kishore-nori !

[kishore-nori]

Hi @askorupka, sorry for bugging you again on this. I have come across the same precompilation error again when CUDA.jl is in the package environment. And I add MPI.jl and Flux.jl. (The error doesn't occur when CUDA.jl is not in the package environment)

ERROR: LoadError: ArgumentError: Package FluxMPIExt does not have CUDA in its dependencies:
- You may have a partially installed environment. Try `Pkg.instantiate()`
  to ensure all packages in the environment are installed.
- Or, if you have FluxMPIExt checked out for development and have
  added CUDA as a dependency but haven't updated your primary
  environment's manifest file, try `Pkg.resolve()`.
- Otherwise you may need to report an issue with FluxMPIExt 

can you check if this occurs for you as well? I would suggest clearing the Flux.jl compilations from .julia folder, to reproduce the error.

[ToucheSir]

I would recommend continuing discussion on #2490 since open issues are more discoverable.