[WIP] EncoderDecoder model that works

[sshleifer]

Continuing #3383 from @patrickvonplaten to facilitate MarianMT project.

Targeted API:

model = EncoderDecoderModel.from_model_names('bert-base-uncased', 'bert-based-uncased')
model.save_pretrained('bert2bert')
model.from_pretrained('bert2bert') # way 2

TODO

• support test_common
• test forward, generate
• raise useful errors for incompatible encoder/decoder combinations