Fix for continuing training
If `args.should_continue` is true, then there is no way in the current example to reload a checkpoint since it is assumed it exists within the output_dir. Checking here fixes everything as expected.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3509?src=pr&el=h1) Report
> Merging [#3509](https://codecov.io/gh/huggingface/transformers/pull/3509?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/601ac5b1dc1438f00d09696588f2deb0f045ae3b&el=desc) will **not change** coverage by `%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3509/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3509?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3509   +/-   ##
=======================================
  Coverage   77.79%   77.79%           
=======================================
  Files         100      100           
  Lines       17051    17051           
=======================================
  Hits        13265    13265           
  Misses       3786     3786           
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3509?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/3509?src=pr&el=footer). Last update [601ac5b...43fda01](https://codecov.io/gh/huggingface/transformers/pull/3509?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
