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.

llm/memory.go

@@ -423,7 +423,7 @@ func projectorMemoryRequirements(filename string) (weights uint64) {
 	}
 	defer file.Close()
 
-	ggml, _, err := ggml.Decode(file, 1024)
+	ggml, err := ggml.Decode(file, 1024)
 	if err != nil {
 		return 0
 	}