Add PerceiverForTokenClassification
# What does this PR do?

You can also use the Perceiver to do named-entity recognition! Training for 3 epochs on CoNLL 2003 gives me the following result (using the official example script):

```
***** eval metrics *****
  epoch                   =        3.0
  eval_LOC_f1             =     0.8895
  eval_LOC_number         =      11884
  eval_LOC_precision      =     0.8934
  eval_LOC_recall         =     0.8856
  eval_MISC_f1            =     0.8058
  eval_MISC_number        =       6078
  eval_MISC_precision     =     0.8182
  eval_MISC_recall        =     0.7938
  eval_ORG_f1             =     0.8215
  eval_ORG_number         =       8869
  eval_ORG_precision      =     0.8187
  eval_ORG_recall         =     0.8243
  eval_PER_f1             =     0.8477
  eval_PER_number         =      10479
  eval_PER_precision      =     0.8598
  eval_PER_recall         =     0.8359
  eval_loss               =     0.1479
  eval_overall_accuracy   =     0.9594
  eval_overall_f1         =      0.848
  eval_overall_precision  =     0.8539
  eval_overall_recall     =     0.8421
  eval_runtime            = 0:01:25.54
  eval_samples            =       3250
  eval_samples_per_second =      37.99
  eval_steps_per_second   =      4.757
```

This PR includes the tweaked example script to make it work for the Perceiver. I can remove it if required, or move it to research-projects directory.

Fixes #14971
cc'ing @Narsil as the token classification pipeline currently doesn't work with the Perceiver. Shoud I remove it from the Auto API?<|||||>@NielsRogge What does happen when it doesn't work ?

Is it taking text only ? then it should be fixable, no ?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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