🐛 T5 Tokenizer ignores \n \t characters and more than one whitespace together
## Environment info
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- `transformers` version: 3.0.2 (master)
- Platform: Linux-4.9.0-12-amd64-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.5.1+cpu (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

### Who can help

@patrickvonplaten @sshleifer

## Information
T5 Tokenizer is based out of SentencePiece and in sentencepiece Whitespace is treated as a basic symbol. But huggingface tokenizers just ignores more than one whitespace. 
Consider all the following examples tokenize to the same thing.
```
from transformers import T5Tokenizer
tokenizer = T5Tokenizer.from_pretrained("t5-base")

print(tokenizer.tokenize("Hi there I'm good"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there     I'm good"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there    I'm good\n"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there \n   I'm good\n"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there \n     I'm good\n"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there \n  \t   I'm good\n"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']

print(tokenizer.tokenize("Hi there\nI'm good"))
>> ['▁Hi', '▁there', '▁I', "'", 'm', '▁good']
```
All these examples should tokenize to different representations. Also ignoring newline outright means that all applications that use newlines fail.
Model I am using (Bert, XLNet ...): T5


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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
All the whitespaces have different tokenizations
@TevenLeScao @sshleifer @patrickvonplaten This looks like a serious problem with the T5 Tokenizer. Is this behavior expected?<|||||>Closing this issue as sentencepiece for T5 removes more than one whitespace as a standard https://github.com/google-research/text-to-text-transfer-transformer/issues/390#issuecomment-688417703