[XLA:GPU] Unify the Redzone buffer creation for autotuning.
[XLA:GPU] Unify the Redzone buffer creation for autotuning.

The three different autotuning passes (cublas, cudnn, triton) have similar code for creating redzone allocators and initializing the buffers. This changes unifies the logic in order to ensure consistency and to make it easier to use this logic in future code.
