transcribe-cli and transcribe-bench never register backend modules: model loads fail with TRANSCRIBE_ERR_BACKEND in a GGML_BACKEND_DL build

[shaba]

In a build configured with -DTRANSCRIBE_GGML_BACKEND_DL=ON, every run of
transcribe-cli or transcribe-bench that loads a model fails with
TRANSCRIBE_ERR_BACKEND.

Root cause

Nothing registers the backend modules. transcribe_init_backends_default() is
called only in list_devices_main() (examples/cli/main.cpp:362), which main()
reaches solely for --list-devices; tools/transcribe-bench/main.cpp never calls
it. With an empty device registry init_cpu_backend() returns nullptr and the
load fails. transcribe-bench also rejects --device N up front, since the index
resolves through the same registry (:334).

examples/hello and examples/hello_stream call transcribe_open() without
registering either.

The comment at tools/transcribe-bench/main.cpp:327 is misleading: it says
--backend is consumed "via transcribe::load_common::init_backends", which
selects a device from the registry but does not populate it.

Reproduction

cmake -B build -DCMAKE_BUILD_TYPE=Release \
      -DTRANSCRIBE_GGML_BACKEND_DL=ON -DTRANSCRIBE_BUILD_SHARED=ON \
      -DTRANSCRIBE_BUILD_EXAMPLES=ON -DTRANSCRIBE_BUILD_TOOLS=ON
cmake --build build -j
cp -a build/bin/libggml-*.so build/src/        # modules next to libtranscribe
export LD_LIBRARY_PATH=$PWD/build/src:$PWD/build/ggml/src

build/bin/transcribe-cli --list-devices        # works: 1 compute device(s)
build/bin/transcribe-bench --model m.gguf --sample a.wav --device 0
# error: --device index 0 is not available

Fix

Calling transcribe_init_backends_default() once at the top of each tool's
main() is enough. Verified against the reproduction above:

--- a/examples/cli/main.cpp
+++ b/examples/cli/main.cpp
@@ -754,6 +754,16 @@ int main(int argc, char ** argv) {
         transcribe_log_set(log_cb, nullptr);
     }
 
+    // Register the backend modules before the first model load. In a
+    // dynamic-backend build nothing is registered until a host asks for it,
+    // and the loader then finds no CPU device and fails the load with
+    // TRANSCRIBE_ERR_BACKEND; in a compiled-in build this is a no-op.
+    if (const transcribe_status st = transcribe_init_backends_default(); st != TRANSCRIBE_OK) {
+        std::fprintf(stderr, "error: transcribe_init_backends_default: %s\n", transcribe_status_string(st));
+        return EXIT_FAILURE;
+    }
+
     std::ofstream   output_file;

The same block goes into tools/transcribe-bench/main.cpp after the log setup
(before --device is resolved), and into the two examples.

[cjpais]

Would you like to submit a PR?