Issue: runing dataset.map, it raises TypeError: can't pickle Tokenizer objects

Comment:
I have the same issue with `transformers/BertJapaneseTokenizer`.



```python
# train_ds = Dataset(features: {
#     'title': Value(dtype='string', id=None), 
#     'score': Value(dtype='float64', id=None)
# }, num_rows: 99999)

t = BertJapaneseTokenizer.from_pretrained('bert-base-japanese-whole-word-masking')
encoded = train_ds.map(lambda examples: {'tokens': t.encode(examples['title'])}, batched=True)
```

<details><summary>Error Message</summary>

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-35-2b7d66b291c1> in <module>
      2 
      3 encoded = train_ds.map(lambda examples:
----> 4   {'tokens': t.encode(examples['title'])}, batched=True)

/usr/local/lib/python3.6/site-packages/datasets/arrow_dataset.py in map(self, function, with_indices, input_columns, batched, batch_size, drop_last_batch, remove_columns, keep_in_memory, load_from_cache_file, cache_file_name, writer_batch_size, features, disable_nullable, fn_kwargs, num_proc, suffix_template, new_fingerprint)
   1242                 fn_kwargs=fn_kwargs,
   1243                 new_fingerprint=new_fingerprint,
-> 1244                 update_data=update_data,
   1245             )
   1246         else:

/usr/local/lib/python3.6/site-packages/datasets/arrow_dataset.py in wrapper(*args, **kwargs)
    151             "output_all_columns": self._output_all_columns,
    152         }
--> 153         out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
    154         if new_format["columns"] is not None:
    155             new_format["columns"] = list(set(new_format["columns"]) & set(out.column_names))

/usr/local/lib/python3.6/site-packages/datasets/fingerprint.py in wrapper(*args, **kwargs)
    156                         kwargs_for_fingerprint["fingerprint_name"] = fingerprint_name
    157                         kwargs[fingerprint_name] = update_fingerprint(
--> 158                             self._fingerprint, transform, kwargs_for_fingerprint
    159                         )
    160 

/usr/local/lib/python3.6/site-packages/datasets/fingerprint.py in update_fingerprint(fingerprint, transform, transform_args)
    103     for key in sorted(transform_args):
    104         hasher.update(key)
--> 105         hasher.update(transform_args[key])
    106     return hasher.hexdigest()
    107 

/usr/local/lib/python3.6/site-packages/datasets/fingerprint.py in update(self, value)
     55     def update(self, value):
     56         self.m.update(f"=={type(value)}==".encode("utf8"))
---> 57         self.m.update(self.hash(value).encode("utf-8"))
     58 
     59     def hexdigest(self):

/usr/local/lib/python3.6/site-packages/datasets/fingerprint.py in hash(cls, value)
     51             return cls.dispatch[type(value)](cls, value)
     52         else:
---> 53             return cls.hash_default(value)
     54 
     55     def update(self, value):

/usr/local/lib/python3.6/site-packages/datasets/fingerprint.py in hash_default(cls, value)
     44     @classmethod
     45     def hash_default(cls, value):
---> 46         return cls.hash_bytes(dumps(value))
     47 
     48     @classmethod

/usr/local/lib/python3.6/site-packages/datasets/utils/py_utils.py in dumps(obj)
    365     file = StringIO()
    366     with _no_cache_fields(obj):
--> 367         dump(obj, file)
    368     return file.getvalue()
    369 

/usr/local/lib/python3.6/site-packages/datasets/utils/py_utils.py in dump(obj, file)
    337 def dump(obj, file):
    338     """pickle an object to a file"""
--> 339     Pickler(file, recurse=True).dump(obj)
    340     return
    341 

/usr/local/lib/python3.6/site-packages/dill/_dill.py in dump(self, obj)
    444             raise PicklingError(msg)
    445         else:
--> 446             StockPickler.dump(self, obj)
    447         stack.clear()  # clear record of 'recursion-sensitive' pickled objects
    448         return

/usr/local/lib/python3.6/pickle.py in dump(self, obj)
    407         if self.proto >= 4:
    408             self.framer.start_framing()
--> 409         self.save(obj)
    410         self.write(STOP)
    411         self.framer.end_framing()

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    474         f = self.dispatch.get(t)
    475         if f is not None:
--> 476             f(self, obj) # Call unbound method with explicit self
    477             return
    478 

/usr/local/lib/python3.6/site-packages/dill/_dill.py in save_function(pickler, obj)
   1436                                 globs, obj.__name__,
   1437                                 obj.__defaults__, obj.__closure__,
-> 1438                                 obj.__dict__, fkwdefaults), obj=obj)
   1439         else:
   1440             _super = ('super' in getattr(obj.func_code,'co_names',())) and (_byref is not None) and getattr(pickler, '_recurse', False)

/usr/local/lib/python3.6/pickle.py in save_reduce(self, func, args, state, listitems, dictitems, obj)
    608         else:
    609             save(func)
--> 610             save(args)
    611             write(REDUCE)
    612 

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    474         f = self.dispatch.get(t)
    475         if f is not None:
--> 476             f(self, obj) # Call unbound method with explicit self
    477             return
    478 

/usr/local/lib/python3.6/pickle.py in save_tuple(self, obj)
    749         write(MARK)
    750         for element in obj:
--> 751             save(element)
    752 
    753         if id(obj) in memo:

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    474         f = self.dispatch.get(t)
    475         if f is not None:
--> 476             f(self, obj) # Call unbound method with explicit self
    477             return
    478 

/usr/local/lib/python3.6/site-packages/dill/_dill.py in save_module_dict(pickler, obj)
    931             # we only care about session the first pass thru
    932             pickler._session = False
--> 933         StockPickler.save_dict(pickler, obj)
    934         log.info("# D2")
    935     return

/usr/local/lib/python3.6/pickle.py in save_dict(self, obj)
    819 
    820         self.memoize(obj)
--> 821         self._batch_setitems(obj.items())
    822 
    823     dispatch[dict] = save_dict

/usr/local/lib/python3.6/pickle.py in _batch_setitems(self, items)
    850                 k, v = tmp[0]
    851                 save(k)
--> 852                 save(v)
    853                 write(SETITEM)
    854             # else tmp is empty, and we're done

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    519 
    520         # Save the reduce() output and finally memoize the object
--> 521         self.save_reduce(obj=obj, *rv)
    522 
    523     def persistent_id(self, obj):

/usr/local/lib/python3.6/pickle.py in save_reduce(self, func, args, state, listitems, dictitems, obj)
    632 
    633         if state is not None:
--> 634             save(state)
    635             write(BUILD)
    636 

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    474         f = self.dispatch.get(t)
    475         if f is not None:
--> 476             f(self, obj) # Call unbound method with explicit self
    477             return
    478 

/usr/local/lib/python3.6/site-packages/dill/_dill.py in save_module_dict(pickler, obj)
    931             # we only care about session the first pass thru
    932             pickler._session = False
--> 933         StockPickler.save_dict(pickler, obj)
    934         log.info("# D2")
    935     return

/usr/local/lib/python3.6/pickle.py in save_dict(self, obj)
    819 
    820         self.memoize(obj)
--> 821         self._batch_setitems(obj.items())
    822 
    823     dispatch[dict] = save_dict

/usr/local/lib/python3.6/pickle.py in _batch_setitems(self, items)
    845                 for k, v in tmp:
    846                     save(k)
--> 847                     save(v)
    848                 write(SETITEMS)
    849             elif n:

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    519 
    520         # Save the reduce() output and finally memoize the object
--> 521         self.save_reduce(obj=obj, *rv)
    522 
    523     def persistent_id(self, obj):

/usr/local/lib/python3.6/pickle.py in save_reduce(self, func, args, state, listitems, dictitems, obj)
    632 
    633         if state is not None:
--> 634             save(state)
    635             write(BUILD)
    636 

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    474         f = self.dispatch.get(t)
    475         if f is not None:
--> 476             f(self, obj) # Call unbound method with explicit self
    477             return
    478 

/usr/local/lib/python3.6/site-packages/dill/_dill.py in save_module_dict(pickler, obj)
    931             # we only care about session the first pass thru
    932             pickler._session = False
--> 933         StockPickler.save_dict(pickler, obj)
    934         log.info("# D2")
    935     return

/usr/local/lib/python3.6/pickle.py in save_dict(self, obj)
    819 
    820         self.memoize(obj)
--> 821         self._batch_setitems(obj.items())
    822 
    823     dispatch[dict] = save_dict

/usr/local/lib/python3.6/pickle.py in _batch_setitems(self, items)
    845                 for k, v in tmp:
    846                     save(k)
--> 847                     save(v)
    848                 write(SETITEMS)
    849             elif n:

/usr/local/lib/python3.6/pickle.py in save(self, obj, save_persistent_id)
    494             reduce = getattr(obj, "__reduce_ex__", None)
    495             if reduce is not None:
--> 496                 rv = reduce(self.proto)
    497             else:
    498                 reduce = getattr(obj, "__reduce__", None)

TypeError: can't pickle Tagger objects
```

</details>

trainsformers: 2.10.0
datasets: 1.0.2
dill: 0.3.2
python: 3.6.8

OS: ubuntu 16.04 (Docker Image) on [Deep Learning VM](https://console.cloud.google.com/marketplace/details/click-to-deploy-images/deeplearning) (GCP)
GPU: Tesla P100 (CUDA 10)