quant : Optimise memory usage by evicting weights after processing each layer (#22877)

* Evict weights from memory after processing each layer

* Revert changes

* Move unmap to libllama

* Unmap weights offloaded to backend

* Change member's constness

* Remove unmap weights offloaded to backend

src/llama-model-loader.cpp

@@ -1395,6 +1395,11 @@ void llama_model_loader::get_mapping_range(size_t * first, size_t * last, void *
     }
 }
 
+void llama_model_loader::unmap_weight(const llama_tensor_weight & w) const {
+    if (!use_mmap) { return; }
+    mappings.at(w.idx)->unmap_fragment(w.offs, w.offs + ggml_nbytes(w.tensor));
+}
+
 void llama_model_loader::load_data_for(struct ggml_tensor * cur) const {
     const auto & w = require_weight(ggml_get_name(cur));