solves several bugs in the summarization codes 
Hi,

This pull requests solves several small bugs in this summarization script:

- call the evaluation code
- fix the iterating over batch in eval part 
- create the folders before saving encoder/decoder to avoid the crash 
- first creates output_dir, then use it during the saving of the model part.
- tokenizer.add_special_tokens_single_sequence does not exist anymore, changed to tokenizer.build_inputs_with_special_tokens
- fix the empty checkpoints in the evaluation 
- add evaluate=False in load_and_cache_examples functions, to fix the issue for now, but dataset needs to be split in train/validation 
- add missing argument of per_gpu_eval_batch_size 

the code needs more features to be added/bugs to be resolved, this just solves the obvious existing bugs, please see my opened bug request #1674 from yesterday on remaining issues. 




# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1694?src=pr&el=h1) Report
> Merging [#1694](https://codecov.io/gh/huggingface/transformers/pull/1694?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/93d2fff0716d83df168ca0686d16bc4cd7ccb366?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1694/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1694?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1694   +/-   ##
=======================================
  Coverage   85.14%   85.14%           
=======================================
  Files          94       94           
  Lines       13920    13920           
=======================================
  Hits        11852    11852           
  Misses       2068     2068
```



------

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<|||||>Thank you for your PR. As I said in the issue, we’re still working on the summarization; you can follow the changes in the ˋexample-summarization` branch.