Fix TFTransfoXLLMHeadModel outputs
# What does this PR do?

Fix the outputs of `TFTransfoXLLMHeadModel` (in the case without `labels`) - current TF returns `softmax_output` while PT returns `prediction_scores`:

- Current PT

    https://github.com/huggingface/transformers/blob/6f9d8dc1567889b073b91059c9b19e9a6813abfa/src/transformers/models/transfo_xl/modeling_transfo_xl.py#L1119
    and

    https://github.com/huggingface/transformers/blob/6f9d8dc1567889b073b91059c9b19e9a6813abfa/src/transformers/models/transfo_xl/modeling_transfo_xl.py#L1138-L1140

- Current TF

    https://github.com/huggingface/transformers/blob/6f9d8dc1567889b073b91059c9b19e9a6813abfa/src/transformers/models/transfo_xl/modeling_tf_transfo_xl.py#L1005-L1006


## Remarks:

  - The case with `labels` is much more complicated - to be addressed in the future.
  - The current PT/TF equivalence test has a bit flaw and doesn't detect this issue. A WIP PR #16557 is on its way (with other enhancements)!
_The documentation is not available anymore as the PR was closed or merged._