[run_seq2seq.py] restore functionality: saving to test_generations.txt (#10428)
[run_seq2seq.py] restore functionality: saving to test_generations.txt (#10428)

This PR restores the original functionality that for some reason was modified.

Fixes: https://github.com/huggingface/transformers/issues/10381

@sgugger
huggingface/pytorch-transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers,huggingface/transformers