Provision of new model(s) for testing multiple model workflows

[jerinphilip]

Need a few more distinct models to test applications enabled by multiple model capability (browsermt/bergamot-translator#209, browsermt/bergamot-translator#210).

It may be desirable to have something compatible with the forward model. Currently there's an archive from here (ende.student.tiny.for.regression.tests) that is kept with separate specs from the usual pool (see #37 (comment)). Wouldn't mind having the same spec as the rest of the models here at the cost of discarding file based shortlists etc.

• One kind of model required is a backward model (de -> en for existing en -> de) so an outbound translation example can be pointed to in code somewhere and runs on CI everytime (For Outbound Translation Requirements bergamot-translator#78)
• Another kind of model we will soon require is (xx->en, en->yy) if and when pivoting (First class pivot translation capability bergamot-translator#212) comes into the picture.

[kpu]

We have en->{et,es,cs} and {et,es,cs}->en. While I understand that generally we need to improve language support, you already have enough for testing purposes.