Preserve defaultdict batches during device placement

[aswanth-07]

What does this PR do?

Preserves standard-library defaultdict batches while send_to_device recursively moves their values.

send_to_device currently reconstructs every mapping as type(mapping)(items). For defaultdict, that positional argument is interpreted as the default factory, so prepared dataloaders crash before yielding a batch. This change rebuilds a defaultdict with both its original default_factory and the moved values, while leaving the existing path unchanged for other mapping types.

Fixes #4154

Validation

• pytest tests/test_utils.py::UtilsTester::test_send_to_device -q - 1 passed
• pytest tests/test_utils.py -q -k 'not test_convert_to_fp32' - 43 passed, 5 skipped
• Full repository ruff check . - passed
• Full repository ruff format --check . - 184 files already formatted
• Public Accelerator.prepare(DataLoader(...)) reproduction - fixed on CPU

The one excluded test invokes torch.compile and fails on this Windows host because the MSVC cl compiler is unavailable; it is unrelated to this mapping change.

Before submitting

• I read the contributor guideline.
• The bug is documented in send_to_device crashes on defaultdict batches #4154.
• No documentation update is needed; this restores nested mapping behavior.
• I added a focused regression test.

AI disclosure: I used Codex to help identify and reproduce the bug, search for duplicates, implement the focused patch, run validation, and draft this PR. I reviewed and understand the change and will personally handle review feedback.