common : fix state save in common_prompt_batch_decode (#23468)

* common : fix state save in common_prompt_batch_decode

This commit addresses a bug in common_prompt_batch_decode that affects
the session state store/restore in completion.cpp and
save-load-state.cpp.

The motivation for this is that currently the code is saving n-1 tokens
in both the session_tokens and in the KV cache. Then when loading the
session tokens, and if the prompt matches, it would replay the last
saved token (n-1) into the next position, effectively replaying the
same token in the wrong position.

The fix is to store all n tokens in session_tokens, while the memory
state only reflects n-1 processed tokens as the saving happens before
the last token is decoded in common_prompt_batch_decode.

I ran both completion.cpp and save-load-state.cpp with a transformer, a
recurrent, and a hybrid model.

Resolves: https://github.com/ggml-org/llama.cpp/issues/23400

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>

common/common.h

@@ -930,7 +930,8 @@ void common_batch_add(
 // tokens from memory, so this approach works across all model architectures.
 bool common_prompt_batch_decode(
               struct llama_context * ctx,
-    const std::vector<llama_token> & embd,
+    const std::vector<llama_token> & all_tokens,
+                               int   n_new,
                                int & n_past,
                                int   n_batch,
                   std::string_view   state_path,