Replace seqeval code with original classification_report for simplicity
Hello,

I ran https://github.com/huggingface/transformers/blob/master/examples/token-classification/run.sh

And received this error:
```
100%|██████████| 407/407 [21:37<00:00,  3.44s/it]Traceback (most recent call last):
  File "run_ner.py", line 445, in <module>
    main()
  File "run_ner.py", line 398, in main
    results = trainer.evaluate()
  File "/data/2021/transformers/src/transformers/trainer.py", line 1470, in evaluate
    metric_key_prefix=metric_key_prefix,
  File "/data/2021/transformers/src/transformers/trainer.py", line 1622, in prediction_loop
    metrics = self.compute_metrics(EvalPrediction(predictions=preds, label_ids=label_ids))
  File "run_ner.py", line 345, in compute_metrics
    results = metric.compute(predictions=true_predictions, references=true_labels)
  File "/usr/local/lib/python3.6/dist-packages/datasets/metric.py", line 398, in compute
    output = self._compute(predictions=predictions, references=references, **kwargs)
  File "/root/.cache/huggingface/modules/datasets_modules/metrics/seqeval/81eda1ff004361d4fa48754a446ec69bb7aa9cf4d14c7215f407d1475941c5ff/seqeval.py", line 97, in _compute
    report = classification_report(y_true=references, y_pred=predictions, suffix=suffix, output_dict=True)
TypeError: classification_report() got an unexpected keyword argument 'output_dict'
```

I'm still trying multiple things to see if I can work around this, but I thought it might be useful to mention it here.

```
Name: transformers
Version: 4.3.0.dev0

Name: datasets
Version: 1.2.1
``` Hi, can you try to update your local installation of `seqeval` ?

```
pip install --upgrade seqeval
``` @lhoestq thanks for the reply. Indeed it was some issue with my setup. I removed the "transformers" and "datasets" (that I had previously installed from the source code), cleared the cache and installed everything again. It works great now!
Recently, the original seqeval has enabled us to get per type scores and overall scores as a dictionary.

This PR replaces the current code with the original function(`classification_report`) to simplify it.

Also, the original code has been updated to fix #352.
- Related issue: https://github.com/chakki-works/seqeval/pull/38


```python
from datasets import load_metric
metric = load_metric("seqeval")
y_true = [['O', 'O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
y_pred = [['O', 'O', 'B-MISC', 'I-MISC', 'I-MISC', 'I-MISC', 'O'], ['B-PER', 'I-PER', 'O']]
metric.compute(predictions=y_pred, references=y_true)
# Output: {'MISC': {'precision': 0.0, 'recall': 0.0, 'f1': 0, 'number': 1}, 'PER': {'precision': 1.0, 'recall': 1.0, 'f1': 1.0, 'number': 1}, 'overall_precision': 0.5, 'overall_recall': 0.5, 'overall_f1': 0.5, 'overall_accuracy': 0.8}
```