add requiredMem method to llmodel impls

most of these can just shortcut out of the model loading logic llama is a bit worse to deal with because we submodule it so I have to at least parse the hparams, and then I just use the size on disk as an estimate for the mem size (which seems reasonable since we mmap() the llama files anyway)

gpt4all-backend/llamamodel_impl.h

@@ -17,6 +17,7 @@ public:
 
     bool loadModel(const std::string &modelPath) override;
     bool isModelLoaded() const override;
+    size_t requiredMem(const std::string &modelPath) override;
     size_t stateSize() const override;
     size_t saveState(uint8_t *dest) const override;
     size_t restoreState(const uint8_t *src) override;