Remove deprecated torch.cuda.amp custom fwd and bwd

[snarayan21]

What does this PR do?

Addresses the deprecation warning:

/usr/lib/python3/dist-packages/turbo/quantized_linear.py:16: FutureWarning: `torch.cuda.amp.custom_fwd(args...)` is deprecated. Please use `torch.amp.custom_fwd(args..., device_type='cuda')` instead.
  @custom_fwd

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