Repository: huggingface/evaluate
Issue: `perplexity` yields `IndexError: index out of range in self`
Root cause: Validation Gap

Small snippet to reproduce the problem:

```python
#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import os

import evaluate
from datasets import load_dataset

os.environ["TOKENIZERS_PARALLELISM"] = "false"

perplexity = evaluate.load("perplexity", module_type="metric")

print('Loading dataset ..')
dataset = load_dataset("JeanKaddour/minipile", "default")
input_texts = dataset["test"]["text"][:10]

print('Computing metrics ..')
results = perplexity.compute(model_id='gpt2',
                             add_start_token=False,
                             predictions=input_texts)

print(round(results["mean_perplexity"], 0))
print(round(results["perplexities"][0], 0))
```

Here is the full trace:

```bash
Traceback (most recent call last):
  File "./snippets/bug.py", line 18, in <module>
    results = perplexity.compute(model_id='gpt2',
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/evaluate/module.py", line 462, in compute
    output = self._compute(**inputs, **compute_kwargs)
  File "/Users/pasquale/.cache/huggingface/modules/evaluate_modules/metrics/evaluate-metric--perplexity/8ab643ad86f568b7d1d5f7822373fa7401ff5ff0297ccf114b0ca6a33be96bc0/perplexity.py", line 179, in _compute
    out_logits = model(encoded_batch, attention_mask=attn_mask).logits
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1518, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1527, in _call_impl
    return forward_call(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 1074, in forward
    transformer_outputs = self.transformer(
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1518, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1527, in _call_impl
    return forward_call(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/transformers/models/gpt2/modeling_gpt2.py", line 838, in forward
    position_embeds = self.wpe(position_ids)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1518, in _wrapped_call_impl
    return self._call_impl(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1527, in _call_impl
    return forward_call(*args, **kwargs)
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/modules/sparse.py", line 162, in forward
    return F.embedding(
  File "/Users/pasquale/anaconda3/lib/python3.8/site-packages/torch/nn/functional.py", line 2233, in embedding
    return torch.embedding(weight, input, padding_idx, scale_grad_by_freq, sparse)
IndexError: index out of range in self
```

Setting `max_length` fixes this

Shall we set `max_length` to `getattr(tokenizer, "model_max_length", getattr(tokenizer, "max_len", None))` automatically if it's `None` or `"auto"`?