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

tools/perplexity/perplexity.cpp

@@ -2023,7 +2023,6 @@ int llama_perplexity(int argc, char ** argv) {
     }
 
     const int32_t n_ctx = params.n_ctx;
-
     if (n_ctx <= 0) {
         LOG_ERR("%s: perplexity tool requires '--ctx-size' > 0\n", __func__);
         return 1;