metal : initial Metal4 tensor API support (llama/16634)

metal : initial Metal4 tensor API support (llama/16634)

* metal : rework mat-mat multiplication

* metal : initial Metal4 support

* cont

* metal : detect tensor support

* cont : better ifdefs

* metal : support tensors in mul_mm_id

* metal : add env for disabling tensor API

* tests : restore

* metal : remove unused constants

* metal : fix check for bfloat tensor support

* cont : handle API incompatibilities

* cont : handle even more incompatibilities

* metal : use tensor API only on M5 and later