Change asserts in src/transformers/models/xlnet/ to raise ValueError (#14088)
* Change asserts in src/transformers/models/xlnet/ to raise ValueError
* Update src/transformers/models/xlnet/modeling_tf_xlnet.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>