fix(tracker): guard init_trackers and log against None kwargs

[xodn348]

Summary

Accelerator.init_trackers and Accelerator.log both declare their optional dict arguments as dict | None, but the function bodies call .get() on the value directly without a None guard. Passing init_kwargs=None or log_kwargs=None therefore raises AttributeError: 'NoneType' object has no attribute 'get' at runtime, even though the type annotation marks None as valid. The root cause is that both signatures used = {} as the default (a mutable-default antipattern in Python) while accepting None in the union type; the fix changes the default to None and adds an explicit if … is None: … = {} guard inside each function body.

Issue

No prior issue — found via code review while auditing tracker-related code paths.

Local verification

cd /tmp/accelerate
pip install -e ".[quality]" -q
pytest tests/test_tracking.py::CustomTrackerTestCase -v

Output:

tests/test_tracking.py::CustomTrackerTestCase::test_init_trackers PASSED
tests/test_tracking.py::CustomTrackerTestCase::test_init_trackers_none_kwargs PASSED
tests/test_tracking.py::CustomTrackerTestCase::test_log PASSED
tests/test_tracking.py::CustomTrackerTestCase::test_log_none_kwargs PASSED

4 passed in 1.54s

=== LOCAL_TEST_PASSED ===

Risk

The change is backward-compatible: callers that already pass a real dict (the common case) see no difference. The only behavior change is that None is now tolerated instead of crashing, which matches what the type annotation already promised. No distributed or multi-GPU code paths are touched.