ggml: new optimization interface (ggml/988)

* ggml: new optimization interface

remove test2.c, test3.c

store adamw params in tensor

move grads from tensor to graph

* avoid segfault upon API misuse

* add ggml-opt.h to public headers

* remove dependence of ggml-opt.cpp on ggml-cpu.h

ggml/src/ggml-backend.cpp

@@ -279,7 +279,7 @@ void ggml_backend_tensor_get(const struct ggml_tensor * tensor, void * data, siz
     buf->iface.get_tensor(buf, tensor, data, offset, size);
 }
 
-GGML_API void ggml_backend_tensor_memset(struct ggml_tensor * tensor, uint8_t value, size_t offset, size_t size) {
+void ggml_backend_tensor_memset(struct ggml_tensor * tensor, uint8_t value, size_t offset, size_t size) {
     ggml_backend_buffer_t buf = tensor->view_src ? tensor->view_src->buffer : tensor->buffer;
 
     if (size == 0) {