[Doctest] - Fixing doctest bert_generation configuration
# What does this PR do?
Fixing doctest in bert_generation configuration. Issue : #19487

## Fixes # (issue)
Added (`with random weights)` in `modeling_ber_generation.py`.
Added `modeling_ber_generation.py` in `documentation_tests.txt`

## Who can review?
@ydshieh @sgugger 



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