Better feature flags for backends.

[nathanielsimard]

I think we should revisit the name of our feature flags and backends. To make things simple we should have:

• tch => libtorch
• wgpu-spirv => vulkan
• wgpu => webgpu
• cuda-jit => cuda

Some feature flags are meant to customize a backend, they should be specific for that backend:

• cuda => candle-cuda, libtorch-cuda
• metal => candle-metal, libtorch-metal
• openblas => ndarray-openblas
• accelerate => ndarray-accelerate

Essentially, we're making backends made with cubecl first class, meaning we don't need to prefix things with cubecl like cubecl-vulkan, cubecl-cuda or cubecl-fusion, but for the third-party backends, I think it's more flexible, since we won't have feature flags that may clash with our own. In that sense, we may also rename burn-jit to burn-cubecl, since it's pretty much the backend built on top of cubecl.

[antimora]

Just to be clear these feature flags are part of the meta package burn?