ggml : add callback to abort ggml_graph_compute() (#328)

ggml : add callback to abort ggml_graph_compute() (#328)

* mechanism to abort ggml_graph_compute

* use pthread_cancel

* forgot to commit ggml.h

* static always_false()

Co-authored-by: Georgi Gerganov <redacted>
* accept callback data

* proper function prototype

* return exit status

* remove pthread_cancel and join every thread

* put abort_callback onto cplan

* cplan abort_callback in ggml.c

* make sure all threads abort

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Co-authored-by: Georgi Gerganov <redacted>