[cli] Uploads: add progress bar
see https://github.com/huggingface/transformers/pull/2044#discussion_r354057827 for context

There might be a more pythonic way (to do a "simple" method overriding) but I couldn't find it.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=h1) Report
> Merging [#2078](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/35ff345fc9df9e777b27903f11fa213e4052595b?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2078/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #2078      +/-   ##
==========================================
+ Coverage   83.16%   83.18%   +0.01%     
==========================================
  Files         109      109              
  Lines       15858    15874      +16     
==========================================
+ Hits        13188    13204      +16     
  Misses       2670     2670
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/hf\_api.py](https://codecov.io/gh/huggingface/transformers/pull/2078/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL2hmX2FwaS5weQ==) | `97.5% <100%> (+0.62%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=footer). Last update [35ff345...5543617](https://codecov.io/gh/huggingface/transformers/pull/2078?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
