Add a dispatch for LinearAlgebra.norm2

`norm(@view x[..], 2)` was previously leading to a call of `LinearAlgebra.generic_norm2` which led to a scalar indexing. This catches such cuda subarray norm2 calls earlier.

Inf-norm and p-norm with cuda subarrays still leads to the following dispatches:
```julia
LinearAlgebra.generic_normInf(x) = float(mapreduce(norm, max, x))
LinearAlgebra.generic_norm1(x) = mapreduce(float ∘ norm, +, x)
```

I am not sure if there is a better way to dispatch them.

lib/cublas/linalg.jl

@@ -138,6 +138,10 @@ function LinearAlgebra.norm(x::DenseCuArray{<:Union{Float16, ComplexF16, CublasF
     end
 end
 
+function LinearAlgebra.norm2(x::SubArray{T,N,P} where {T<:Union{Float16, ComplexF16, CublasFloat}, N, P<:DenseCuArray{<:T}})
+    return nrm2(x)
+end
+
 LinearAlgebra.BLAS.asum(x::StridedCuArray{<:CublasFloat}) = asum(length(x), x)
 
 function LinearAlgebra.axpy!(alpha::Number, x::StridedCuArray{T}, y::StridedCuArray{T}) where T<:Union{Float16, ComplexF16, CublasFloat}

test/libraries/cublas.jl

@@ -1767,6 +1767,20 @@ end
             @view(p[reshape(1:(out*inn),out,inn)]) * x
         end
     end
+
+    @testset "nrm2 with strided inputs" begin # JuliaGPU/CUDA.jl#2280
+        cudaTypes = (Float16, Complex{Float16}, BFloat16, Complex{BFloat16}, Float32, Complex{Float32},
+                    Float64, Complex{Float64}, Int8, Complex{Int8}, UInt8, Complex{UInt8},
+                    Int16, Complex{Int16}, UInt16, Complex{UInt16}, Int32, Complex{Int32},
+                    UInt32, Complex{UInt32}, Int64, Complex{Int64}, UInt64, Complex{UInt64})
+        for CT in cudaTypes
+            x = rand(CT, 10, 10, 10)
+            dx = CuArray(x)
+            dx_ = @view dx[3:6, 1:5, :]
+            x_ = @view x[3:6, 1:5, :] 
+            @test norm(dx_, 2) ≈ norm(x_, 2)
+        end
+    end
 end
 
 ############################################################################################