transformers-cli upload individual files simplification
Author: stas00
Pull request: no

Currently it's not possible to upload an individual file in a simple:
```
transformers-cli upload fsmt-wmt19-ru-en/vocab-src.json 
```
Getting error:
```
Filename invalid, every file must be nested inside a "model_name" folder.
```
but, instead, have to do:
```
transformers-cli upload fsmt-wmt19-ru-en/vocab-src.json --filename fsmt-wmt19-ru-en/vocab-src.json
```
But this is silly as the exact same input is repeated twice and I'm more likely to make an error while copy-n-pasting when providing an explicit destination filename. Why not look at the relative path and use that? And only give that error when there is no "model_name" folder in the args. i.e. definitely give error on:
```
transformers-cli upload vocab.json
```
I understand the ` --filename` is useful for renaming, but there is no renaming here.

Additionally, if the first suggestion is acceptable, would it be OK to support multiple filenames? I want to be able to update several files in one go (just the config files, but not the whole folder, since the model is huge)

```
transformers-cli upload fsmt-wmt19-ru-en/vocab-*.json 
```
gives error:
```
Transformers CLI tool: error: unrecognized arguments: fsmt-wmt19-ru-en/vocab-ru.json fsmt-wmt19-ru-en/vocab-src.json fsmt-wmt19-ru-en/vocab-tgt.json
```
Thanks.

Comments:
Well, I wrote a little script to generate the long commands that are required now - perhaps it'd be useful to someone:
```
perl -le 'for $f (@ARGV) { print qq[yes Y | transformers-cli upload $_/$f --filename $_/$f] for map { "fsmt-wmt19-$_" } ("en-ru", "ru-en", "de-en", "en-de")}' vocab-src.json vocab-tgt.json tokenizer_config.json
```

generated:
```
yes Y | transformers-cli upload fsmt-wmt19-en-ru/vocab-src.json --filename fsmt-wmt19-en-ru/vocab-src.json
yes Y | transformers-cli upload fsmt-wmt19-ru-en/vocab-src.json --filename fsmt-wmt19-ru-en/vocab-src.json
yes Y | transformers-cli upload fsmt-wmt19-de-en/vocab-src.json --filename fsmt-wmt19-de-en/vocab-src.json
yes Y | transformers-cli upload fsmt-wmt19-en-de/vocab-src.json --filename fsmt-wmt19-en-de/vocab-src.json
yes Y | transformers-cli upload fsmt-wmt19-en-ru/vocab-tgt.json --filename fsmt-wmt19-en-ru/vocab-tgt.json
yes Y | transformers-cli upload fsmt-wmt19-ru-en/vocab-tgt.json --filename fsmt-wmt19-ru-en/vocab-tgt.json
yes Y | transformers-cli upload fsmt-wmt19-de-en/vocab-tgt.json --filename fsmt-wmt19-de-en/vocab-tgt.json
yes Y | transformers-cli upload fsmt-wmt19-en-de/vocab-tgt.json --filename fsmt-wmt19-en-de/vocab-tgt.json
yes Y | transformers-cli upload fsmt-wmt19-en-ru/tokenizer_config.json --filename fsmt-wmt19-en-ru/tokenizer_config.json
yes Y | transformers-cli upload fsmt-wmt19-ru-en/tokenizer_config.json --filename fsmt-wmt19-ru-en/tokenizer_config.json
yes Y | transformers-cli upload fsmt-wmt19-de-en/tokenizer_config.json --filename fsmt-wmt19-de-en/tokenizer_config.json
yes Y | transformers-cli upload fsmt-wmt19-en-de/tokenizer_config.json --filename fsmt-wmt19-en-de/tokenizer_config.json
```

As I have an easy workaround that works well, unless others feel the suggested improvements in the OP would be useful, I'd be happy to close this ticket.

pinging @julien-c 

@julien-c, Should this be closed or fixed? Thanks.

Closing as we are migrating to a new system anyways (more info soon)