Add AMDGPU extension

[pxl-th]

This PR adds initial support for AMDGPU via extension mechanism.

PR Checklist

• Tests are added (will add more in a bit).
• Entry in NEWS.md
• Documentation, if applicable

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• Add FLUX_TEST_AMDGPU env variable to specify whether to run AMDGPU tests or not.
• Add ability to switch between GPU backends using Preferences.jl.
  E.g. Flux.gpu_backend!("AMD") or gpu_backend="AMD" in LocalPreferences.toml allows specifying which backend to use.
• Handle regular convolution, since MIOpen only supports cross-correlation as convolution.
  Therefore I plan to specify adaptor for convolution for Flux.amd which will flip weights during conversion once.
  Not sure if this is optimal way, so if anyone has suggestions do let me know :)
• Use MIOpen for BatchNorm.

[pxl-th]

Updated to the latest NNlib which includes AMDGPU extension.
Added documentation on AMDGPU as well.
Should be good to go if there are no complains

[ToucheSir]

Looks real good, just a few touch-up comments from me. What are your thoughts on CI? Should we wait until the AMDGPU runners are stable before adding a Buildkite pipeline?

[pxl-th]

Looks real good, just a few touch-up comments from me. What are your thoughts on CI? Should we wait until the AMDGPU runners are stable before adding a Buildkite pipeline?

I think we can wait until we know the CI is working and then add it here.
NNlib has configurations for AMD so we will see it there

[ToucheSir]

Ok, let's do this. Thanks @pxl-th !