ggml: Seperate tensor load from backend creation

Currently, when the backend is created, the tensors are loaded at the
same time, which is a slow operation. This separates them to be two
steps:
 - Create backend, including enumerating tensors and memory allocation
 - Loading tensor data

This allows more flexibility in managing model loading.

server/images.go

@@ -75,7 +75,7 @@ func (m *Model) Capabilities() []model.Capability {
 	if err == nil {
 		defer r.Close()
 
-		f, _, err := ggml.Decode(r, 1024)
+		f, err := ggml.Decode(r, 1024)
 		if err == nil {
 			if _, ok := f.KV()[fmt.Sprintf("%s.pooling_type", f.KV().Architecture())]; ok {
 				capabilities = append(capabilities, model.CapabilityEmbedding)