When encoding text to feature vectors - Would be awesome to be able to use the simplest tokenizer with a split on spaces
# 🚀 Feature request

When using the `DistilBertTokenizer` (or `BertTokenizer`) I would love to tokenize my text by simply splitting it on spaces:
```
['also', 'du', 'fängst', 'an', 'mit', 'der', 'Stadtrundfahrt']
```
instead of the default behavior, which is splitting it into sub-parts:
```
['also', 'du', 'f', '##ängst', 'an', 'mit', 'der', 'Stadt', '##rund', '##fahrt']
```


## Motivation
That's needed in order to have feature vector length to be the same as the number of words in the text, so that we can have 1-to-1 correspondence between words and their features

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     related to a problem? e.g., I'm always frustrated when [...]. If this is related
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## Your contribution

I have seen that such tokenization can be done using `tokenizer.basic_tokenizer` and I have tried to use it to encode the text:
```
from transformers import DistilBertModel, DistilBertTokenizer
import torch

text_str = "also du fängst an mit der Stadtrundfahrt"

# create DistilBERT tokenizer and model
tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-german-cased')
model = DistilBertModel.from_pretrained('distilbert-base-german-cased')

# check if tokens are correct
tokens = tokenizer.basic_tokenizer.tokenize(text_str)
print("Tokens: ", tokens)

# Encode the curent text
input_ids = torch.tensor(tokenizer.basic_tokenizer.encode(text_str)).unsqueeze(0)
outputs = model(input_ids)
last_hidden_states = outputs[0]
```

But this code raises an error because `BasicTokenizer` does not have attribute 'encode' yet:
```
Traceback (most recent call last):
  File "/home/tarask/Desktop/Work/Code/Git/probabilistic-gesticulator/my_code/data_processing/annotations/feat/test.py", line 15, in <module>
    input_ids = torch.tensor(tokenizer.basic_tokenizer.encode(text_str)).unsqueeze(0)
AttributeError: 'BasicTokenizer' object has no attribute 'encode'
```

This solution seems to be working:
```
from transformers import DistilBertModel, DistilBertTokenizer
import torch

text_str = "also du fängst an mit der Stadtrundfahrt"

# create DistilBERT tokenizer and model
tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-german-cased')
model = DistilBertModel.from_pretrained('distilbert-base-german-cased')

# check if tokens are correct
tokens = tokenizer.basic_tokenizer.tokenize(text_str)
print("Tokens: ", tokens)

# Encode the curent text
input_ids = torch.tensor(tokenizer.encode(tokens)).unsqueeze(0)
outputs = model(input_ids)
last_hidden_states = outputs[0]
print(last_hidden_states[0,1:-1].shape)
```

What do you think? Is it correct usage?<|||||>I just realized it was not correct. [Users of StackOverflow](https://stackoverflow.com/questions/66064503/in-huggingface-tokenizers-how-can-i-split-a-sequence-simply-on-spaces/) indicated that.

Running the following command: `tokenizer.convert_ids_to_tokens(input_ids.tolist()[0])` indicates that "fängst" and "Stadtrundfahrt" are encoded with the same id because they are not part of the dictionary :( <|||||>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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