[ASR] Add missing Python 3 type hints to transcribe_utils.py

[shivansh023023]

What does this PR do?

Adds complete Python 3 type annotations to public functions in
nemo/collections/asr/parts/utils/transcribe_utils.py, as required by
CONTRIBUTING.md: "Use Python 3 type hints for every class and method exposed to the user."

Collection: ASR

Changelog

• Added Dict to the existing typing imports
• get_buffered_pred_feat_rnnt: manifest, filepaths, target_lang_id → Optional[...]
• get_buffered_pred_feat_multitaskAED: manifest, filepaths → Optional[...]
• restore_transcription_order: return type list → Union[List, Tuple]
• normalize_timestamp_output: timestamps: dict → Dict, added -> Dict return type
• write_transcription: filepaths → Optional[List[str]]
• compute_metrics_per_sample: output_manifest_path → Optional[str]
• PunctuationCapitalization.__init__: added -> None return type

Usage

No API changes. Type hints are purely additive and improve IDE autocompletion
and static analysis for downstream users subclassing or calling these utilities.

Before your PR is "Ready for review"

Pre checks:

• Make sure you read and followed Contributor guidelines
• Did you write any new necessary tests? — No tests needed, annotations only
• Did you add or update any necessary documentation? — No docs changes needed
• Does the PR affect components that are optional to install? — No

PR Type:

• New Feature
• Bugfix
• Documentation / Code Style

Additional Information

• No logic changes whatsoever — annotations only
• black and isort checks pass locally
• Follows the same pattern as merged PR Add typing to speech_to_text_finetune.py #15326 which added typing to speech_to_text_finetune.py

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