mtmd: release/restore the encoder weights from VRAM on demand

Add clip_release_device/clip_restore_device (and mtmd_release_device/
mtmd_restore_device wrappers over the vision + audio contexts) that free
the multimodal encoder's device weight buffer to a read-only host shadow
and rebuild it on demand, using the same shadow/free/reallocate pattern
as llama_model weights. No-op for a CPU-backed encoder. This lets the
server drop the ~hundreds-of-MiB vision encoder from VRAM when it is not
encoding an image.

Assisted-by: Claude

tools/mtmd/mtmd.cpp

@@ -814,6 +814,24 @@ void mtmd_free(mtmd_context * ctx) {
     delete ctx;
 }
 
+void mtmd_release_device(mtmd_context * ctx) {
+    if (ctx == nullptr) {
+        return;
+    }
+    if (ctx->ctx_v) { clip_release_device(ctx->ctx_v); }
+    if (ctx->ctx_a) { clip_release_device(ctx->ctx_a); }
+}
+
+bool mtmd_restore_device(mtmd_context * ctx) {
+    if (ctx == nullptr) {
+        return true;
+    }
+    bool ok = true;
+    if (ctx->ctx_v) { ok = clip_restore_device(ctx->ctx_v) && ok; }
+    if (ctx->ctx_a) { ok = clip_restore_device(ctx->ctx_a) && ok; }
+    return ok;
+}
+
 struct mtmd_tokenizer {
     mtmd_context * ctx;