add -y to bypass prompt for transformers-cli upload
As discussed here: https://github.com/huggingface/transformers/issues/6934#issuecomment-687365960
adding `-y`/`--yes` to bypass prompt for scripting.

<!-- This line specifies which issue to close after the pull request is merged. -->
Fixes #6934

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7035?src=pr&el=h1) Report
> Merging [#7035](https://codecov.io/gh/huggingface/transformers/pull/7035?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/76818cc4c6a1275a23ba261ca337b9f9070c397e?el=desc) will **decrease** coverage by `0.36%`.
> The diff coverage is `16.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7035/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7035?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7035      +/-   ##
==========================================
- Coverage   79.43%   79.06%   -0.37%     
==========================================
  Files         164      164              
  Lines       31026    31028       +2     
==========================================
- Hits        24645    24532     -113     
- Misses       6381     6496     +115     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7035?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/commands/user.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb21tYW5kcy91c2VyLnB5) | `36.73% <16.66%> (+0.18%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9sb25nZm9ybWVyLnB5) | `16.37% <0.00%> (-82.31%)` | :arrow_down: |
| [src/transformers/modeling\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sb25nZm9ybWVyLnB5) | `19.71% <0.00%> (-72.34%)` | :arrow_down: |
| [src/transformers/configuration\_longformer.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xvbmdmb3JtZXIucHk=) | `75.00% <0.00%> (-25.00%)` | :arrow_down: |
| [src/transformers/modeling\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190NS5weQ==) | `76.70% <0.00%> (-6.07%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `86.66% <0.00%> (-0.55%)` | :arrow_down: |
| [src/transformers/modeling\_bert.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19iZXJ0LnB5) | `88.28% <0.00%> (-0.17%)` | :arrow_down: |
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `94.04% <0.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `86.97% <0.00%> (+1.62%)` | :arrow_up: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `86.71% <0.00%> (+7.51%)` | :arrow_up: |
| ... and [3 more](https://codecov.io/gh/huggingface/transformers/pull/7035/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7035?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/7035?src=pr&el=footer). Last update [76818cc...8feb8ec](https://codecov.io/gh/huggingface/transformers/pull/7035?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>LGTM (this could lead to more invalidly-named checkpoints being uploaded, but we'll soon have more server-side validation)