metal : single allocation of encode_async block (#9747)

* Single allocation of encode_async block with non-ARC capture in ggml-metal.m

* Moving Block_release to the deallocation code

* Release encode block when re-setting encoding buffer count if needed

* Update ggml/src/ggml-metal.m

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>