Transformer-XL: Remove unused/unnecessary Parameters
# 🚀 Feature request

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The used configuration parameters `tgt_len` and `ext_len` in the Transformer-XL implementation are not utilized in the source code. I would recommend removing those since the only confuse the users of the model(s).

## Motivation

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As already mentioned, the unnecessary parameters are confusing and can be likely removed. The parameter `tgt_len` is determined for the input tensor anyway and `ext_len` was only an experimental feature (see [issue from the original repo](https://github.com/kimiyoung/transformer-xl/issues/9)).

## Your contribution

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I am happy to contribute and open a PR with the requested changes if that's also in your interest?
ping @TevenLeScao 