Modernize type hints in ignite/metrics/loss.py to use PEP 604 union syntax and built-in generics, aligning with Python 3.10+ (now minimum supported version).

Changes

• Union[str, torch.device] → str | torch.device
• Tuple[...] → tuple[...]
• Dict → dict
• Improved update() signature from Sequence[Union[torch.Tensor, Dict]] to tuple[torch.Tensor, torch.Tensor] | tuple[torch.Tensor, torch.Tensor, dict] for better type safety
• Removed obsolete typing imports

# Before
def update(self, output: Sequence[Union[torch.Tensor, Dict]]) -> None:
    if len(output) == 2:
        y_pred, y = cast(Tuple[torch.Tensor, torch.Tensor], output)
        kwargs: Dict = {}

# After  
def update(self, output: tuple[torch.Tensor, torch.Tensor] | tuple[torch.Tensor, torch.Tensor, dict]) -> None:
    if len(output) == 2:
        y_pred, y = cast(tuple[torch.Tensor, torch.Tensor], output)
        kwargs: dict = {}

Part of ongoing effort to modernize typing across the codebase.

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