Scale buf_size linearly with n_ctx

This appear to solve ggml-org#153
where error of "ggml_new_tensor_impl: not enough space in the context's memory pool" is thrown in interactive mode. 

At least the out of memory error come from `ctx0` used here. Although I am not familiar with the code base enough to tell if this is indeed the cause.

Author: hx507

main.cpp

@@ -549,9 +549,7 @@ bool llama_eval(
 
     const int d_key = n_embd/n_head;
 
-     // TODO: check if this size scales with n_ctx linearly and remove constant. somehow I feel it wasn't the case
-    // static size_t buf_size = hparams.n_ctx*1024*1024;
-    static size_t buf_size = 512u*1024*1024;
+    static size_t buf_size = (size_t)hparams.n_ctx*1024*1024;
     static void * buf = malloc(buf_size);
 
     if (mem_per_token > 0 && mem_per_token*N > buf_size) {