ElectraTokenizerFast
Before the fix, when loading an `ElectraTokenizerFast`:

```py
from transformers import ElectraTokenizerFast

tokenizer = ElectraTokenizerFast.from_pretrained("ahotrod/electra_large_discriminator_squad2_512")
```
```
Traceback (most recent call last):
  File "/Users/jik/Library/Application Support/JetBrains/PyCharm2020.2/scratches/7735.py", line 3, in <module>
    tokenizer = ElectraTokenizerFast.from_pretrained("ahotrod/electra_large_discriminator_squad2_512")
  File "/Users/jik/Workspaces/python/transformers/src/transformers/tokenization_utils_base.py", line 1555, in from_pretrained
    resolved_vocab_files, pretrained_model_name_or_path, init_configuration, *init_inputs, **kwargs
  File "/Users/jik/Workspaces/python/transformers/src/transformers/tokenization_utils_base.py", line 1623, in _from_pretrained
    tokenizer = cls(*init_inputs, **init_kwargs)
  File "/Users/jik/Workspaces/python/transformers/src/transformers/tokenization_bert.py", line 641, in __init__
    **kwargs,
  File "/Users/jik/Workspaces/python/transformers/src/transformers/tokenization_utils_fast.py", line 89, in __init__
    self._tokenizer = convert_slow_tokenizer(slow_tokenizer)
  File "/Users/jik/Workspaces/python/transformers/src/transformers/convert_slow_tokenizer.py", line 565, in convert_slow_tokenizer
    converter_class = CONVERTERS[transformer_tokenizer.__class__.__name__]
KeyError: 'ElectraTokenizer'
```