gemma4_31b (MLX): don't allocate a second KV-cache copy for single-session runs (#20560)

Since #20473, the MLX engine always enables the multi-session
mutable-state path, so create_session() allocates a per-session KV-cache
copy on top of the program's default buffers — which rebind never uses
again once a session exists — leaving single-session runs holding two
full KV caches. This gates the MLX mutable_state creation on
config.max_sessions > 1, so the single-session CLI runner (max_sessions
= 1) executes against the default buffers (one KV cache, as before
#20473) while the multi-session worker is unchanged. Result: lower peak
memory and recovered prefill tok/s for single-session runs (significant
at long context); decode is unaffected. Test: make gemma4_31b-mlx then
run the runner — peak memory drops and prefill recovers, multi-session
isolation still works.

Follow-up: This only fixes the single-session case. Under multi-session
the program's default mutable buffers are still allocated but go unused
once any session is created, so the worker pays one extra dead KV-cache
copy (N+1 for N sessions). A follow-up could release or donate the
default buffers to the first session under multi-session, with a check
that the init chain only zero-initializes KV state (per-session buffers
are freshly zeroed and don't replay it).

Author: metascroy

examples/models/gemma4_31b/gemma4_31b_engine.cpp

@@ -684,7 +684,15 @@ Result<std::unique_ptr<Gemma4_31BEngine>> Gemma4_31BEngine::create(
     }
   }
 #elif defined(EXECUTORCH_BUILD_MLX)
-  mutable_state = std::make_unique<GemmaMutableStateContextOwner>();
+  // Only enable the per-session mutable-buffer path when actually serving more
+  // than one session. For a single session (the CLI runner) the rebind would
+  // allocate a second copy of the KV-cache buffers on top of the program's
+  // default buffers — doubling KV-cache memory and adding a one-time
+  // session-buffer allocation during the first prefill — for no isolation
+  // benefit. Leaving mutable_state null keeps the program's default buffers.
+  if (config.max_sessions > 1) {
+    mutable_state = std::make_unique<GemmaMutableStateContextOwner>();
+  }
 #endif
 
   auto module_res = mutable_state != nullptr