Fix eigenvalue parsing for compression-only quantize configs

[sowndappan5]

Description

This PR fixes eigenvalue configuration parsing in deepspeed/runtime/config.py for quantization-enabled configs.

Previously, get_eigenvalue_config() assumed the legacy top-level quantize_training section always existed after get_quantize_enabled() returned true. That caused a KeyError for supported compression configs that enable weight quantization through compression_training without also defining quantize_training.

This change makes the parser fall back to the default eigenvalue settings when the legacy section is absent, while preserving the existing behavior when quantize_training is present.

Testing

Added a regression test covering a compression_training weight-quantization config without a legacy quantize_training section.

Validated locally with the repository's focused pre-commit checks.

[chatgpt-codex-connector]

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