Fix TopkRetriever metadata collation

[hongleng]

Motivation

Fixes #1686.

DataCollatorWithPaddingAndCuda currently adds non-tensor metadata to the
BatchEncoding before calling .to(device).

In Transformers 4.46.2, BatchEncoding.to() retains only tensor values.
Therefore, the metadata wrapper is removed during device transfer, and
TopkRetriever.forward() later raises KeyError: 'metadata'.

Modification

• Keep metadata as its original ordered list instead of wrapping it in
  ListWrapper.
• Move the padded tensor batch to the target device before attaching metadata.
• Remove the no-longer-needed ListWrapper.
• Add a regression test covering both:
  • the legacy behavior that calls .to() on every batch value;
  • the tensor-only behavior used by newer Transformers versions.

The test also verifies that the collator still returns a single
BatchEncoding, tensor fields remain unchanged, and metadata content and
ordering are preserved.

BC-breaking (Optional)

No backward-incompatible changes are introduced.

The collator continues to return a single BatchEncoding, and
TopkRetriever.forward() continues to require the metadata field. The fix
has been verified with Transformers 4.41.0, 4.46.2, and 4.57.6.

Tests

• pytest tests/openicl/test_icl_topk_retriever.py -q
  • 1 passed, 2 subtests passed
• pytest tests/openicl -q
  • 53 passed, 2 subtests passed
• flake8, isort, yapf, codespell, and the other applicable pre-commit hooks
  passed for the modified files

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, and the case that causes the bug is included.
• The modification is covered by unit tests.
• Documentation update is not required because this does not change the public API or user-facing behavior.

After PR:

• Compatibility has been checked with Transformers 4.41.0, 4.46.2, and 4.57.6.
• CLA has been signed and all committers have signed the CLA in this PR.