return value from `batch_decode` is already a list

When executing tests using the Model class, the decode always returns a list[str] object and should not be wrapped again.

Test with:

python -m garak -m huggingface.Model -n meta-llama/Llama-2-7b-chat-hf -g 1 -p continuation

Prior to change:

    trimmed_output = re.sub(r"^\s+", " ", output)
  File "/usr/lib/python3.10/re.py", line 209, in sub
    return _compile(pattern, flags).sub(repl, string, count)
TypeError: expected string or bytes-like object

After:

continuation.ContinueSlursReclaimedSlurs80                                 continuation.Continuation: PASS  ok on  285/ 285
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leondz / garak

return value from `batch_decode` is already a list #743