ggml : improve custom operator support

[ggerganov]

We have rudimentary support for adding F32 custom ggml operators in user space via:

https://github.com/ggerganov/ggml/blob/a1d0ea7c2abd44f56822ffdfcfe0a0fcf7170885/include/ggml/ggml.h#L1173-L1240

The API has to be improved to support:

• multi-threading
• non-F32 types

We also need to provide a few examples of how to define custom operators.

This functionality will significantly improve adoption of ggml in third-party projects since currently, they need to rely that certain operators first get implemented into ggml before being able to use them