context : do not re-reserve the scheduler when toggling causal_attn (#28751)

* context : do not re-reserve the scheduler when toggling causal_attn

`llama_context::set_causal_attn()` marks the scheduler to do a full re-reserve on every change of the flag. For vision inputs, this flag is flipped twice around each non-causal image chunk for Gemma models, resulting in two expensive `sched_reserve()` passes per image. This is especially slow for multi-image or video inputs.

The cost of a re-reserve scales with context and ubatch configurations, so larger settings pay more per image (see table below).

The re-reserve is unnecessary in this case because `causal_attn` only changes the values written to KQ mask, not tensor shapes or any other buffer sizes.

Note: `causal_attn` is a graph reuse key (`llm_graph_params` via `cparams`), so a new graph is built regardless of `sched_need_reserve`, so this doesn't change the graph rebuilding behaviour.

llama-server with gemma-4-26B-A4B Q4_0 + BF16 mmproj, 130-token images,
cache_prompt=false, prompt_ms median of 3 (before -> after):

| images | config | H200 before -> after | RTX 4090 before -> after |
|-|-|-|-|
| 1 | `-c 8192 -ub 512` | 134 -> 105 ms (1.27×) | 201 -> 119 ms (1.69×) |
| 24 | `-c 8192 -ub 512` | 2278 -> 1562 ms (1.46×) | 3559 -> 1748 ms (2.04×) |
| 24 | `-c 32768 -ub 2048` | 5379 -> 1584 ms (3.40×) | 13377 -> 1759 ms (7.61×) |

Generated output remains identical before and after.

* qwen4exp : make the indexer bias shape independent of causal_attn

The block/cell bias path was selected on cparams.causal_attn, so the
causal and non-causal graphs differed in tensor shapes and ops. With the
re-reserve removed (previous commit), a runtime flip resulted in
reallocating the compute buffers, which would fail under
GGML_SCHED_NO_REALLOC.

This commit selects the block path from the mask shape only, independent
of causal_attn. causal_attn is instead passed to set_input_qsa.
causal_attn is fixed per graph as it's part of the reuse key. Causal
values are unchanged. Non-causal values now follow the reference rule,
where every visible block competes on score and only unpooled cells are
always selected.

* context : state the causal_attn shape rule in the comment

* cont : add TODOs

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

src/llama-context.cpp

@@ -1256,7 +1256,8 @@ void llama_context::set_causal_attn(bool value) {
 
     cparams.causal_attn = value;
 
-    sched_need_reserve = true;
+    // no scheduler reserve needed because graph shapes must not depend on causal_attn, a flip only rebuilds the graph
+    //sched_need_reserve = true;
 }
 
 bool llama_context::get_causal_attn() const {