SpeechLM2 SALM: load ckpt faster, with less GPU memory

[pzelasko]

What does this PR do ?

Loads speechlm2 checkpoints faster and requiring 4x less peak GPU memory during init. Previously a SALM ckpt that required ~20GB during init, now only requires ~5GB.

Collection: speechlm2

Changelog

• For checkpoints converted to safetensors, skips loading original weights of the pretrained ASR module
• Safetensors checkpoints are read into CPU first, converted to bfloat16, and then put on CUDA to minimize memory usage. After the change, only ~5GB RAM is required for a 2.5B bfloat16 SALM model. Eventually we might want to optimize this further with low-RAM direct GPU loading.

Usage

• You can potentially add a usage example below

# Add a code snippet demonstrating how to use this 

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[zhehuaichen]

Very nice thanks!

[github-actions]

[🤖]: Hi @pzelasko 👋,

We wanted to let you know that a CICD pipeline for this PR just finished successfully.

So it might be time to merge this PR or get some approvals.

//cc @chtruong814 @ko3n1g @pablo-garay @thomasdhc