metal : add memory pool for temp allocs (#12850)

* metal : add memory pool for temp allocs (wip) [no ci]

* cont : free buffers from the heap

* cont : resize heap [no ci]

* cont : refactor heap [no ci]

* cont : heap for each cmd buffer [no ci]

* cont : fix free

* wip

* cont : fix alignment [no ci]

* cont : not working .. [no ci]

* cont : heap allocation now works [no ci]

* cont : use MTLHeapTypePlacement

ggml-ci

* metal : use dynamic MTLHeap allocations

ggml-ci

* metal : add comments

* metal : disable softmax use of mem_pool

ggml-ci

* metal : final touches

Author: ggerganov