Make sure RepeatTeacher doesn't leak files.

[stephenroller]

Right now, running the unittest suite results in a leaking file unused_paths.lengths. This behavior derives from:

ParlAI/tests/test_train_model.py

Line 46 in 5899c07

task='tasks.repeat:RepeatTeacher:10',

ParlAI/tests/tasks/repeat.py

Lines 16 to 26 in 5899c07

	class RepeatTeacher(DialogTeacher):
	def __init__(self, opt, shared=None):
	opt = copy.deepcopy(opt)
	opt['datafile'] = 'unused_path'
	task = opt.get('task', 'tests.tasks.repeat:RepeatTeacher:50')
	self.data_length = int(task.split(':')[2])
	super().__init__(opt, shared)
	def setup_data(self, unused_path):
	for i in range(self.data_length):
	yield ((str(i), [str(i)]), True)

ParlAI/parlai/core/teachers.py

Line 814 in 5899c07

length_file = datafiles[0] + ".lengths"