[assistants] - fix: make assistant overrides optional

[JulesBelveze]

Summary

• allow assistants to use their default data configuration
• preserve caller-provided override dictionaries
• keep assistant names stable during configuration merging

Tests

• uv run pre-commit run --files bert_squeeze/assistants/train_assistant.py bert_squeeze/assistants/distil_assistant.py tests/assistants/test_assistant_defaults.py
• uv run pytest -q tests/assistants/test_assistant_defaults.py tests/assistants/test_train_assistant.py::TestTrainAssistant::test_custom_logger_kwargs tests/assistants/test_distil_assistant.py::TestDistilAssistant::test_custom_logger_kwargs tests/assistants/test_distil_assistant.py::TestDistilAssistant::test_torch_models