Avoid multiple warnings for loss mask

[mikolajblaz]

What does this PR do ?

Currently loss masking for GPT (used when model.data.pad_samples_to_global_batch_size=True) issues a warning for every masked sample. In typical workflows there might be hundreds of masked samples in 1 batch, which spawns hundreds of warnings every validation. This PR fixes that

Collection: NLP

Changelog

• Turn loss mask warning into a debug log

Usage

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[ericharper]

LGTM. Thanks!