Change asserts in src/transformers/models/xlnet/ to raise ValueError

[westonkl]

What does this PR do?

Replaces asserts in:
src/transformers/models/xlnet/configuration_xlnet.py and src/transformers/models/xlnet/modeling_tf_xlnet.py
to a ValueError exception.

Contributes towards fixing issue #12789

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@sgugger

[sgugger]

Thanks for your help!