Make directory at start of dict.save()

[spencerp]

Patch description
If self.bpe.copy_codecs_file is hit before the directory is created, a "path does not exist" exception will be thrown. This won't happen until validation, which can waste a lot of time.

Testing steps

parlai train_model -t convai2 --model transformer/generator --dict-file /path/to/existing/model.dict --model-file /path/to/new/model --embedding-size 32 --n-positions 32 --truncate 32 --validation-every-n-epochs 0.01 --dict-tokenizer bpe

Where /path/to/existing/model.dict exists, but /path/to/new/ doesn't exist yet.

[klshuster]

good catch, lgtm thanks!