[tsnAnh]

Summary

Fixes a Metal debug validation crash in Gather::eval_gpu when gather/take uses scalar indices.

Gather already has a scalar-index specialization (idx_ndim == 0), but the Metal encoder still receives empty index metadata vectors:

compute_encoder.set_vector_bytes(idx_shapes, 7);
compute_encoder.set_vector_bytes(idx_strides, 8);
compute_encoder.set_vector_bytes(idx_contigs, 9);

With Metal API validation enabled, an empty vector reaches setBytes:length:attributeStride:atIndex: as a nil bytes argument and aborts.

Evidence

Observed downstream crash:

-[MTLDebugComputeCommandEncoder setBytes:length:attributeStride:atIndex:]:387: failed assertion `bytes argument cannot be nil.'
Task: signal SIGABRT

The abort was isolated to this line in mlx/backend/metal/indexing.cpp:

compute_encoder.set_vector_bytes(idx_shapes, 7);

The same file already handles this exact Metal binding edge case in Scatter::eval_gpu by pushing dummy metadata when idx_ndim == 0:

if (idx_ndim == 0) {
  // Add a 0 in idx_shapes and strides to avoid the missing buffer binding
  // error in the metal API.
  idx_shapes.push_back(0);
  idx_strides.push_back(0);
  idx_contigs.push_back(false);
}

This PR applies the same guard to Gather::eval_gpu. idx_ndim remains 0, so the scalar-index specialization behavior does not change; the patch only ensures Metal receives non-nil metadata buffers.

Existing public API coverage already includes scalar-index take:

out = mx.take(a, mx.array(1), axis=0)
self.assertEqual(out.shape, (4,))

Validation

Ran locally:

git diff --check

Downstream validation after applying the same patch in a SwiftPM MLX checkout:

xcodebuild -quiet -project iosApp/iosApp.xcodeproj -scheme iosApp -configuration Debug -destination 'generic/platform=iOS Simulator' -skipMacroValidation -skipPackagePluginValidation build
xcodebuild -quiet -project iosApp/iosApp.xcodeproj -scheme iosApp -configuration Debug -destination 'generic/platform=iOS' -skipMacroValidation -skipPackagePluginValidation build
./gradlew :shared:linkDebugFrameworkIosSimulatorArm64

Those builds passed and no longer hit the Metal nil-buffer assertion.