Added capability to quantize a model while exporting through ONNX.
Author: mfuntowicz
Pull request: yes

Add quantization support as part of our collaboration with ONNX team.

Quantization is available through the new method `quantize` which takes the path to the initial ONNX model and operate the conversion.

From a CLI point of view, adding `--quantize` allows to seamlessly export the quantized model.

Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>

Comments:
cc: @tianleiwu @yufenglee from Microsoft

# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=h1) Report
> Merging [#6089](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/640550fc7a1e311915ead1bcca6dacea0c503faf&el=desc) will **increase** coverage by `0.68%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/6089/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #6089      +/-   ##
==========================================
+ Coverage   77.85%   78.53%   +0.68%     
==========================================
  Files         146      146              
  Lines       26326    26326              
==========================================
+ Hits        20496    20676     +180     
+ Misses       5830     5650     -180     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_t5.py](https://codecov.io/gh/huggingface/transformers/pull/6089/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl90NS5weQ==) | `45.98% <0.00%> (-44.92%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6089/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.98% <0.00%> (-0.98%)` | :arrow_down: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6089/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `82.20% <0.00%> (-0.29%)` | :arrow_down: |
| [src/transformers/generation\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/6089/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9nZW5lcmF0aW9uX3RmX3V0aWxzLnB5) | `80.70% <0.00%> (+0.75%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_mobilebert.py](https://codecov.io/gh/huggingface/transformers/pull/6089/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9tb2JpbGViZXJ0LnB5) | `96.77% <0.00%> (+73.38%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=footer). Last update [640550f...16ecccc](https://codecov.io/gh/huggingface/transformers/pull/6089?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).