Expose device enumeration

[nazar-pc]

I was looking for ways to enumerate available devices (with wgpu, but also applicable for other backends), but didn't find it.
Enumeration is used internally here:

cubecl/crates/cubecl-wgpu/src/runtime.rs

Lines 397 to 398 in d4e896a

	instance
	.enumerate_adapters(backend.into())

Would be great to expose it in some way publicly. In fact you could make WgpuDevice something that is returned by that list and not constructable explicitly for better misuse resistance.

[Use-AIrs]

Me to for cuda, using cudarc now, but as it seems this could use a trait implementations

[Use-AIrs]

Will add gpu_count() to Runtime as starting point.