Fix: replace assert statements with exceptions in file src/transformers/models/lxmert/modeling_lxmert.py

[murilo-goncalves]

What does this PR do?

Replaced assert statements with ValueError exceptions in file src/transformers/models/lxmert/modeling_lxmert.py,
as discussed in issue #12789

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@sgugger

[sgugger]

Thanks a lot!