[Reformer] remove reformer pad_token_id
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

The `crime-and-punishment` tokenizer actually does not have a `pad_token_id` - check with this [notebook](https://colab.research.google.com/github/google/trax/blob/master/trax/models/reformer/text_generation.ipynb#scrollTo=iDgvKNa_DDIq). Since this is our only tokenizer for Reformer, we should remove the `pad_token` completely from the Reformer Tokenizer script (otherwise `tokenizer.pad_token_id` get's an id >= tokenizer.max_len`).

Since `crime-and-punishment` runs on causal attention, any token can be set to the padding token during inference. 
Thus before padding one should do `tokenizer.pad_token = tokenizer.eos_token`.

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Fixes #7929


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