metal : make the backend async (#15906)

metal : make the backend async (#15906)

* metal : make the backend async

ggml-ci

* cont : add comments, extend op offload, clean up

ggml-ci

* metal : fix batch size for MUL_MAT_ID

* metal : remove deprecated ggml_backend_metal_buffer_from_ptr

* metal : create only metal buffers, no wrapping of host memory

ggml-ci

* metal : restore .alloc_buffer for buffer_from_ptr_type

ggml-ci

* metal : remove broken implementation of GGML_OP_SET

ggml-ci

* metal : clean-up loose ends, ready for tests

ggml-ci

* metal : support both private and shared buffers

ggml-ci

* metal : enable private buffers + add global device queue

* metal : disable host buffer to prevent races

ggml-ci

* metal : avoid extra copy during set_tensor

ggml-ci

* metal : use separate buffer types for shread and private Metal buffers

ggml-ci

* metal : simplify synchronization logic

ggml-ci

* metal : fix build

ggml-ci

* metal : do not implement cpy_tensor

ggml-ci

* metal : separate implementations for shared and private buffers

ggml-ci