ggml : add metal backend registry / device

[ggerganov]

target #9707

Adapt the Metal backend to the new registry and device interfaces.

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[slaren]

ggml_backend_metal_buffer_type() also needs to be updated to set the device field.

[slaren]

There have been a few minor changes to the interfaces:

• The get_backend_reg function of the device interface has been removed, instead a pointer is stored directly in ggml_backend_device: https://github.com/ggerganov/llama.cpp/pull/9707/commits/d0c4954fa01e90f61f9b5c5849f0d5732df0ea9f
• Some functions have been renamed: https://github.com/ggerganov/llama.cpp/pull/9707/commits/cfef355611e38db20dedd9faad5f7b29e14a3004

[slaren]

This seems to be working now.

[ggerganov]

Should we put a deprecate notice for these API calls?

https://github.com/ggerganov/llama.cpp/blob/6dcb8991704b40d923691f037cdecc5430ff0440/ggml/include/ggml-metal.h#L41-L51

[slaren]

Should we put a deprecate notice for these API calls?

I think it may be too early for that, it's probably better to wait a bit until all the backends and the ggml examples are updated.