feat(trainer): log epoch and final metrics
This PR adds new logging capabilities to `Trainer`:

* logs epoch -> this let us compare runs with different batch sizes, using "epoch" as the x-axis instead of "global_step"
* logs final metrics -> metrics are also logged by `Trainer.evaluate` at the end of the script (or at the start if we just use `--do_eval` and not train).

In below example, we see `epoch` has been used for x-axis and `eval_loss` has one extra point corresponding to the end of the script.
![image](https://user-images.githubusercontent.com/715491/81350982-d57e5600-9088-11ea-8ce6-400bda09bea7.png)
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/4221?src=pr&el=h1) Report
> Merging [#4221](https://codecov.io/gh/huggingface/transformers/pull/4221?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/c99fe0386be118bceaab1c85cdb8309eb8cb8208&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `39.39%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/4221/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/4221?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4221      +/-   ##
==========================================
+ Coverage   78.31%   78.34%   +0.02%     
==========================================
  Files         120      120              
  Lines       19854    19864      +10     
==========================================
+ Hits        15549    15562      +13     
+ Misses       4305     4302       -3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/4221?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/trainer.py](https://codecov.io/gh/huggingface/transformers/pull/4221/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmVyLnB5) | `42.73% <39.39%> (+2.55%)` | :arrow_up: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/4221/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `92.77% <0.00%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/4221?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/4221?src=pr&el=footer). Last update [c99fe03...f220d8f](https://codecov.io/gh/huggingface/transformers/pull/4221?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Already merged through PR #4324