Use absolute path in scripts

[gonglinyuan]

The script with relative paths raises an error of "No such file or directory: data-bin/wikitext-103/dict.txt" in fairseq-hydra-train.

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• [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
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• [x] Did you make sure to update the docs?
• [x] Did you write any new necessary tests?

What does this PR do?

The script with relative paths raises an error of "No such file or directory: data-bin/wikitext-103/dict.txt" in fairseq-hydra-train.

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[JeanKaddour]

Thanks, this saved me some time!