Update deprecated torch.cuda.amp API calls for PyTorch 2.0+ compatibility

[lksrz]

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• I have searched the YOLOv6 issues and found no similar feature requests.

Description

YOLOv6 currently uses deprecated PyTorch API calls for automatic mixed precision (AMP) training. This causes deprecation warnings when using PyTorch 2.0 or newer.

When training with PyTorch 2.0+, the following warnings appear:
FutureWarning: torch.cuda.amp.GradScaler(args...) is deprecated. Please use torch.amp.GradScaler('cuda', args...) instead.
FutureWarning: torch.cuda.amp.autocast(args...) is deprecated. Please use torch.amp.autocast('cuda', args...) instead.

Proposed Solution
Update the following code patterns throughout the codebase:

• Replace torch.cuda.amp.GradScaler() with torch.amp.GradScaler('cuda')
• Replace torch.cuda.amp.autocast() with torch.amp.autocast('cuda')

This change maintains the same functionality while making the code compatible with PyTorch 2.0+ and eliminating deprecation warnings.

Files that need changes

• yolov6/core/engine.py
• yolov6/models/losses/loss.py

I'm happy to contribute a PR for this change if desired.

Use case

No response

Additional

No response

Are you willing to submit a PR?

• Yes I'd like to help by submitting a PR!

[roxroxroxrox]

good job. i dont update my stack so frequently. from biz to alg.

[ktz03]

I'd like to take this. Plan: replace deprecated torch.cuda.amp.GradScaler/autocast with torch.amp.GradScaler/autocast('cuda') for PyTorch 2.0+ compatibility. PR shortly.

[ktz03]

Opened PR: #1089