Complete revamp of model loading to allow for more discreet control by

the user of the models loading behavior.

Signed-off-by: Adam Treat <treat.adam@gmail.com>

gpt4all-backend/llmodel.h

@@ -74,6 +74,8 @@ public:
         int32_t n_last_batch_tokens = 0;
     };
 
+    using ProgressCallback = std::function<bool(float progress)>;
+
     explicit LLModel() {}
     virtual ~LLModel() {}
 
@@ -125,6 +127,8 @@ public:
     virtual bool hasGPUDevice() { return false; }
     virtual bool usingGPUDevice() { return false; }
 
+    void setProgressCallback(ProgressCallback callback) { m_progressCallback = callback; }
+
 protected:
     // These are pure virtual because subclasses need to implement as the default implementation of
     // 'prompt' above calls these functions
@@ -153,6 +157,15 @@ protected:
 
     const Implementation *m_implementation = nullptr;
 
+    ProgressCallback m_progressCallback;
+    static bool staticProgressCallback(float progress, void* ctx)
+    {
+        LLModel* model = static_cast<LLModel*>(ctx);
+        if (model && model->m_progressCallback)
+            return model->m_progressCallback(progress);
+        return true;
+    }
+
 private:
     friend class LLMImplementation;
 };