Adding deprecation warning to native NeMo RETRO

[huvunvidia]

What does this PR do ?

Adding deprecation warning to native NeMo RETRO

Changelog

Adding deprecation warnings to:

• examples/nlp/language_modeling/megatron_retro_eval_legacy.py
• examples/nlp/language_modeling/megatron_retro_pretraining_legacy.py
• nemo/collections/nlp/models/language_modeling/megatron_retrieval_model.py

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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• New Feature
• Bugfix
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Additional Information

• Related to # (issue)

[ericharper]

LGTM. Thanks!

[github-actions]

This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days.