Remove get_accelerator().amp() API and use torch.amp directly
The amp() method on each accelerator returned a device-specific
torch.<device>.amp module, but since PyTorch 2.4 the unified torch.amp
API (torch.amp.custom_fwd, torch.amp.custom_bwd, torch.amp.autocast)
accepts a device_type argument and works across all backends. The
previous commit already migrated the two call sites; this commit removes
the now-unused amp() abstract method and all 8 accelerator
implementations, plus simplifies the custom_fwd/custom_bwd setup in
zero/linear.py by dropping the pre-2.4 fallback path.
Signed-off-by: Ma, Guokai <guokai.ma@intel.com>
