Handle negative values for dim input in pad_across_processes

[mariusarvinte]

What does this PR do?

Fixes #3113 by explicitly checking and handling padding on negative dimensions for accelerate.utils.pad_across_processes.

Adds tests for handling valid and out-of-bounds negative dimensions.

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@muellerzr @BenjaminBossan @SunMarc

[muellerzr]

Thanks for adding this functionality and a test! :)

[SunMarc]

Nice way to handle this! LGTM!