[t5 tokenizer] add info logs
This PR (was modified from the original):
- adds info logs that correlated to saved tokenizer files on  `tokenizer.save_pretrained()`

--------------------------
original PR note

This PR
- adds code to save t5 fast tokenizer `tokenizer.json` file on `tokenizer.save_pretrained()`
- adds info logs that correlated to saved tokenizer files on  `tokenizer.save_pretrained()`

Context:
- I needed to create a new t5 smallish model and the created model won't work w/o `tokenizer.json`. 
- Also as I was debugging why I was missing that file, I enabled logging and saw that we were getting logs for every saved file, but tokenizer files, so this PR fixes that, so it's consistent and helps one to see if something is missing.

Here is an example:
```
TRANSFORMERS_VERBOSITY=info PYTHONPATH=/hf/transformers-master/src python t5-make-very-small-model.py
[....]
Configuration saved in t5-very-small-random/config.json
Model weights saved in t5-very-small-random/pytorch_model.bin
Configuration saved in t5-very-small-random/config.json
tokenizer config file saved in t5-very-small-random/tokenizer_config.json
Special tokens file saved in t5-very-small-random/special_tokens_map.json
Copy vocab file to t5-very-small-random/spiece.model
tokenizer config file saved in t5-very-small-random/tokenizer_config.json
Special tokens file saved in t5-very-small-random/special_tokens_map.json
Copy vocab file to t5-very-small-random/spiece.model
Copy tokenizer file to t5-very-small-random/tokenizer.json
```

I'm not sure why I needed to save both:
```
tokenizer.save_pretrained(mname_very_small)
tokenizer_fast.save_pretrained(mname_very_small)
```
note `tokenization_t5.py` doesn't have it! both t5 tokenizer files:
```
VOCAB_FILES_NAMES = {"vocab_file": "spiece.model"}
VOCAB_FILES_NAMES = {"vocab_file": "spiece.model", "tokenizer_file": "tokenizer.json"}
```

As I flagged on slack `https://huggingface.co/sshleifer/t5-tinier-random` fails to be used since it's missing this fast `tokenizer.json` file from the s3 set of files,
```
Traceback (most recent call last): 
 File "./finetune_trainer.py", line 373, in <module> 
   main() 
 File "./finetune_trainer.py", line 205, in main 
   tokenizer = AutoTokenizer.from_pretrained( 
 File "/home/stas/hf/transformers/src/transformers/models/auto/tokenization_auto.py", line 385, in from_pretrained 
   return tokenizer_class_fast.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs) 
 File "/home/stas/hf/transformers/src/transformers/tokenization_utils_base.py", line 1768, in from_pretrained 
   return cls._from_pretrained( 
 File "/home/stas/hf/transformers/src/transformers/tokenization_utils_base.py", line 1841, in _from_pretrained 
   tokenizer = cls(*init_inputs, **init_kwargs) 
 File "/home/stas/hf/transformers/src/transformers/models/t5/tokenization_t5_fast.py", line 139, in __init__ 
   super().__init__( 
 File "/home/stas/hf/transformers/src/transformers/tokenization_utils_fast.py", line 86, in __init__ 
   fast_tokenizer = TokenizerFast.from_file(fast_tokenizer_file)
```

it could be a symptom for another problem in our code.


@LysandreJik, @sgugger 
I don't think this is something that should be done in `save_vocabulary`. You have the option in `save_pretrained` to set `legacy_format` to `False` to generate that `tokenizer.json` file. I'm not an expert in the tokenization side with all the stuff that was added for backward compatibility so I don't know if there is a better option.

I wasn't aware havin this file was mandatory for some models to use the fast tokenizer. Are you sure you have sentencepiece installed? It might be due to this that the conversion slow to fast does not work automatically

Anyhow, once we have found the right way to generate that `tokenizer.json` file, it should be added on the model sharing doc page, next to the section on how to generate TF/PyTorch checkpoints, so that people know what to do to have the most complete model on the hub.<|||||>I don't have a problem to add it anywhere else, who do we tag on this?

1. Let the code speak for itself:
```
python -c "from transformers import T5Tokenizer, T5TokenizerFast; mname_from='sshleifer/t5-tinier-random'; tokenizer = T5Tokenizer.from_pretrained(mname_from); tokenizer_fast = T5TokenizerFast.from_pretrained(mname_from)"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/mnt/disc1/data/trash/src/transformers/src/transformers/tokenization_utils_base.py", line 1762, in from_pretrained
    return cls._from_pretrained(
  File "/mnt/disc1/data/trash/src/transformers/src/transformers/tokenization_utils_base.py", line 1835, in _from_pretrained
    tokenizer = cls(*init_inputs, **init_kwargs)
  File "/mnt/disc1/data/trash/src/transformers/src/transformers/models/t5/tokenization_t5_fast.py", line 139, in __init__
    super().__init__(
  File "/mnt/disc1/data/trash/src/transformers/src/transformers/tokenization_utils_fast.py", line 86, in __init__
    fast_tokenizer = TokenizerFast.from_file(fast_tokenizer_file)
Exception: No such file or directory (os error 2)
```

2. If `footokenizer.from_pretrained()` fetches `tokenizer.json` then `footokenizer.save_pretrained()` must save it too.
 
> I wasn't aware havin gthis file was mandatory for some models to use the fast tokenizer. Are you sure you have sentencepiece installed? It might be due to this that the conversion slow to fast does not work automatically

```
pip install sentencepiece
Requirement already satisfied: sentencepiece in /mnt/nvme1/anaconda3/envs/main-38/lib/python3.8/site-packages (0.1.91)
```

If you look at the trace it is hunting for that file and can't find it.

> Anyhow, once we have found the right way to generate that tokenizer.json file, it should be added on the model sharing doc page, next to the section on how to generate TF/PyTorch checkpoints, so that people know what to do to have the most complete model on the hub.

Agreed!

@LysandreJik, @n1t0 <|||||>ok, so as @sgugger suggested on slack, the fast tokenizer saving will be handled on the core-level some time in the future, so I removed that part from this PR, leaving just the logger part.