ggml-backend v2 : add ggml_backend_sched (#586)

ggml-backend v2 : add ggml_backend_sched (#586)

* ggml-backend-v2 wip

* fix metal build

* ggml-alloc : use a real backend buffer in measure mode

* backend sched : ignore view ops to reduce the number of splits

* dynamic ggml_cgraph wip

* dyn graphs : remove n_tasks from ggml_cplan

* dyn graphs : update ggml_graph_import

* reset hash table in ggml_build_forward

* ggml-alloc : split into tensor and graph allocators

* add ggml_backend_sched_set_node_backend

* remove ggml_build_forward_ctx, ggml_build_backward_ctx
add ggml_opt_params::graph_size
add ggml_new_graph_custom, ggml_graph_overhead_custom
add ggml_graph_clear

* update examples and tests, fix issues

* update more examples

* update gpt-2/main-backend.cpp from master

* ggml : fix copmile warning

* ci : update yolo, fix mnist, use gpt-2-backend

* ggml : fix uninit warning

* ci : switch to gpt-2-backend2

ggml-ci

* metal : skip noops early to avoid warnings from ggml_metal_get_buffer

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Co-authored-by: Georgi Gerganov <redacted>