llama : add early return for empty range (#8327)

* llama : add early return for empty range

This commit adds an early return to the llama_kv_cache_seq_add and
llama_kv_cache_seq_div functions.

The motivation for adding this is to avoid looping over the cache
when the range is empty. I ran into this when using the self-extend
feature in main.cpp.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* llama : add static_cast to fix CI warning/error

This commit attempts to fix the following warning/error:

```console
src/llama.cpp:7271:31: error:
comparison of integer expressions of different signedness:
‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Werror=sign-compare]
 7271 |                         if (i < hparams.n_layer_dense_lead) {
      |                             ~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
This can be reproduced locally by setting -Wsign-compare in the
Makefile.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* squash! llama : add early return for empty range

Remove the setting of cache.head to 0 when the range is empty.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

* Update src/llama.cpp

---------

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

src/llama.cpp

@@ -3260,6 +3260,8 @@ static void llama_kv_cache_seq_add(
 
     if (p0 < 0) p0 = 0;
     if (p1 < 0) p1 = std::numeric_limits<llama_pos>::max();
+    // If there is no range then return early to avoid looping over the cache.
+    if (p0 == p1) return;
 
     if (cache.recurrent) {
         // for Mamba-like models, only the pos needs to be shifted
@@ -3304,6 +3306,8 @@ static void llama_kv_cache_seq_div(
                           int   d) {
     if (p0 < 0) p0 = 0;
     if (p1 < 0) p1 = std::numeric_limits<llama_pos>::max();
+    // If there is no range then return early to avoid looping over the cache.
+    if (p0 == p1) return;
 
     if (cache.recurrent) {
         // for Mamba-like models, only the pos needs to be changed