Add bleu metric

[sdesrozis]

Description:

Add BLEU metric. Refactor to introduce nlp module in metrics. Rouge metric has been moved in nlp.

The implementation is inspired from nltk.

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

Looks good @sdesrozis , thanks
I have few comments here and
do we need empty ignite/metrics/nlp/utils.py here ?

[gucifer]

Looks good @sdesrozis , thanks
I have few comments here and
do we need empty ignite/metrics/nlp/utils.py here ?

The utils.py file holds the modified_precision function used in compute_corpus method line 131. Maybe the method has not been moved to utils.py yet. Also, this file can be used to store common nlp pre-processing which we can provide as options when calculating ROUGE or BLEU or other metrics in the future.
Also, categorizing metrics into sub-modules sounds like a good idea in my opinion as in the future the number of metrics supported by ignite will only grow.

[sdesrozis]

Looks good @sdesrozis , thanks

I have few comments here and

do we need empty ignite/metrics/nlp/utils.py here ?

This file should not be empty!! Let me see.

EDIT ok I don't know why this file was not committed correctly. I do it asap.

[sdesrozis]

@vfdev-5 did you ever face flaky codecov failure ?

[vfdev-5]

@sdesrozis yes, codecov is start failing today

[trsvchn]

I think we have the same issue as here codecov/codecov-action#234

[vfdev-5]

Thanks for great PR @sdesrozis 💪 LGTM

You can merge it once CI is done.