Fix WMT dataset loading issue and docs update (Re-opened)
_The documentation is not available anymore as the PR was closed or merged._
This PR is a fix for #4354 

Changes are made for `wmt14`, `wmt15`, `wmt16`, `wmt17`, `wmt18`, `wmt19` and `wmt_t2t`. And READMEs are updated for the corresponding datasets.

Let me know, if any additional changes are required.

Thanks