Replace assert statements with raise exceptions
Author: miyu386
Pull request: yes

# What does this PR do?
Fixes an instance of https://github.com/huggingface/transformers/issues/12789.

Replaces 4 assert statements with ValueError exception in `src/transformers/data/metrics/squad_metrics.py`
Co-author: @mollerup23

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger Appreciate the feedback! I've addressed and made the necessary changes.