Improve logging message in feature conversion functions
This PR adds the total number of examples to process to the log message produced during the feature conversion step.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2361?src=pr&el=h1) Report
> Merging [#2361](https://codecov.io/gh/huggingface/transformers/pull/2361?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f75bf05ce6a05ef316363de129c29f2e00cacd7b?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/2361/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2361?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2361   +/-   ##
=======================================
  Coverage   73.23%   73.23%           
=======================================
  Files          87       87           
  Lines       14985    14985           
=======================================
  Hits        10975    10975           
  Misses       4010     4010
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2361?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/data/processors/utils.py](https://codecov.io/gh/huggingface/transformers/pull/2361/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvdXRpbHMucHk=) | `19.6% <0%> (ø)` | :arrow_up: |
| [src/transformers/data/processors/glue.py](https://codecov.io/gh/huggingface/transformers/pull/2361/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9kYXRhL3Byb2Nlc3NvcnMvZ2x1ZS5weQ==) | `27.86% <0%> (ø)` | :arrow_up: |

------

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<|||||>Great, thanks @simonepri 