Add English-Spanish translation problem

[voluntadpear]

Hi!

I was able to successfully add and use a English-Spanish translation problem, using Common Crawl, EuroParl v7, UN Corpus and ParaCrawl as my datasets.

I trained both an EN-ES and an ES-EN Transformer models (using the transformer_big_single_gpu hyperparameters set) with this new problem specification.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[voluntadpear]

Done CLI

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[lukaszkaiser]

Great thanks, looks good!

[lukaszkaiser]

Did you manage to get good results from the models? Great thanks for contributing!

[iamyihwa]

Hello! Thanks for adding en-es module and making it available!

I have ran into an issue when I tried to download data using 't2t-datagen' command.

export PROBLEM=translate_enes_wmt32k
t2t-datagen --data_dir=$DATA_DIR --tmp_dir=$TMP_DIR --problem=$PROBLEM

It works when I use english to german module for example by doing

export PROBLEM=translate_ende_wmt32k

Some part of the error message that I get clearly show that spanish module is not part of the available one at the moment. (there is no enes module appearing where it should be since things seem to be ordered alphabetically.)

 ValueError: You must specify one of the supported problems to generate data for: * algorithmic_addition_binary40 ... * translate_ende_wmt_clean_pc_clean32k * translate_ende_wmt_multi64k * translate_ende_wmt_multi64k_packed1k * translate_ende_wmt_pc32k * translate_ende_wmt_pc_clean32k * translate_enet_wmt32k * translate_enet_wmt_characters * translate_enfr_wmt32k 

I have tried installing through 'pip install tensor2tensor' as well as 'pip install git+https://github.com/tensorflow/tensor2tensor.git' to check if the last changes were not reflected for the official release.