fix(asr): restore legacy boosting-tree config compatibility

[rohan-patnaik]

What does this PR do ?

Fills fields missing from legacy boosting-tree DictConfig values with current structured defaults before reading them. This restores checkpoint compatibility without disabling struct validation. Fixes #15658.

Collection: ASR

Changelog

• Normalize legacy boosting-tree configs at emptiness checks and model construction.
• Cover configs missing one field or several fields while preserving saved overrides.

Before your PR is "Ready for review"

Pre checks:

• Read and followed the contributor guidelines.
• Added regression tests; 25 adjacent CPU tests also pass (one Triton-only skip).
• No documentation update is needed because this restores existing checkpoint-loading behavior.
• No optional-install component is affected.

PR Type:

• Bugfix

Additional Information

Fixes #15658.

[copy-pr-bot]

This pull request requires additional validation before any workflows can run on NVIDIA's runners.

Pull request vetters can view their responsibilities here.

Contributors can view more details about this message here.