Fix MT dataset targets
This MR uses a dict instead of a list so that target ids greater than the length of the list (caused by missing/skipped target ids, e.g., as in the global_voices dataset) do not produce an error.
This MR uses a dict instead of a list so that target ids greater than the length of the list (caused by missing/skipped target ids, e.g., as in the global_voices dataset) do not produce an error.