Reuse reduction/dot buffer as sqrt output in linalg.norm

[antonwolfy]

In the dpnp.linalg.norm implementation, the 2-norm and Frobenius-norm branches computed sqrt(sum(...)) (or sqrt(dot(...)) on the axis=None fast path) allocating a fresh array for the sqrt output on top of the array already produced by the reduction.

These branches now capture the reduction (or dot) result and pass it back as out= to dpnp.sqrt, so the intermediate buffer is reused as the output instead of allocating a new one. The reused buffer is a private, unaliased array in every case, and its dtype matches the sqrt output, so the in-place write is safe. On the axis=None fast path the reused buffer is a scalar, so the change there is for consistency.

This is an allocation saving only; it does not change results or reduce the number of kernel launches.

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