Fix AMP autocast deprecation warning for PyTorch>=2.5

[moronizacmel]

Summary

Fixes FutureWarning raised by PyTorch>=2.5 when using torch.cuda.amp.autocast,
while preserving backward compatibility with older PyTorch versions.

Changes

• Adds a backward-compatible autocast helper
• Replaces direct imports of torch.cuda.amp.autocast
• No behavior change intended

Compatibility

• No API changes
• No new dependencies

Motivation

PyTorch deprecated torch.cuda.amp.autocast in favor of torch.amp.autocast("cuda").
This PR removes warning spam without breaking older installations.

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