fix(asr): mask padded frames after feature normalization

[arushidNV]

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What does this PR do ?

Fixes a buffered ASR normalization bug where padded frames in short final chunks were normalized using valid-frame statistics and could blow up to large values, causing hallucinations.

Collection: ASR

Changelog

• Update normalize_batch() in nemo/collections/asr/parts/preprocessing/features.py
• Keep existing valid-frame mean/std computation unchanged
• Zero out invalid / padded frames after normalization using the existing valid-frame mask
• Prevent padded tail values from corrupting encoder input for partial last chunks

Usage

• No API change is required

# Add a code snippet demonstrating how to use this 

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Additional Information

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