[fsmt convert script] fairseq broke chkpt data - fixing that
This PR's main purpose:

* [x] adjusts for breaking fairseq [changes](https://github.com/pytorch/fairseq/commit/3b27ed7996b0315f471c795cf9b7dfcc18467cbe)

Plus:

* [x] adds support for older `bpecodes` filenames - specifically `code` in iwslt14
* [x] improves reporting
* [x] removes CDN note that will be shortly become irrelevant https://github.com/huggingface/transformers/pull/8324

The break was manifesting during conversion in:
```
Traceback (most recent call last):
  File "src/transformers/convert_fsmt_original_pytorch_checkpoint_to_pytorch.py", line 271, in <module>
    convert_fsmt_checkpoint_to_pytorch(args.fsmt_checkpoint_path, args.pytorch_dump_folder_path)
  File "src/transformers/convert_fsmt_original_pytorch_checkpoint_to_pytorch.py", line 118, in convert_fsmt_checkpoint_to_pytorch
    src_lang = args["source_lang"]
KeyError: 'source_lang'
```

@sshleifer, @LysandreJik 
+1 It would be good to document supported/tested versions, but I think it would be useful to try to keep up to date (to the extent possible.) I think most people choose whether to upload to hf **after** they have trained a model, and simply won't upload if their version if it is not supported. (Rather than re-train with a supported `fairseq` version.) <|||||>@stas00 You could also write an integration test that runs only if fairseq is installed (probably in examples), that tests conversion. This wouldn't get run by CI, but might make trouble-shooting easier.<|||||>Well, in this case, they haven't changed the output for the model - they changed their hub tools instead (i.e. how the model is loaded) - so no re-training is necessary.  So requiring a fixed fairseq model would probably be sufficient.

The problem is that they stopped making releases last Dec. https://github.com/pytorch/fairseq/tags How can we tell a developer to install version X if it doesn't exist (or check for it)?

I would leave it as is for now, since so far there is barely any interest in fsmt and should it start getting more traction I'll be all over it making it super-resilient. <|||||>Makes sense, feel free to merge @LysandreJik 