Simplify recognize digits example code

[kexinzhao]

Paddle/python/paddle/fluid/trainer.py

Lines 257 to 272 in b67ce35

	def _test_by_executor(self, reader, feed_order, fetch_list):
	with executor.scope_guard(self.scope):
	feed_var_list = build_feed_var_list(self.test_program, feed_order)
	feeder = data_feeder.DataFeeder(
	feed_list=feed_var_list, place=self.place)
	exe = executor.Executor(self.place)
	accumulated = len(fetch_list) * [0]
	count = 0
	for data in reader():
	outs = exe.run(program=self.test_program,
	feed=feeder.feed(data),
	fetch_list=fetch_list)
	accumulated = [x[0] + x[1][0] for x in zip(accumulated, outs)]
	count += 1
	return [x / count for x in accumulated]

The code above returns a list of float scalars to test_metrics as follows:
test_metrics = trainer.test(xxx)

Thus if we use avg_cost, acc = trainer.test(xxx) instead, both avg_cost and acc would be a Python float scalar.

Hence we don't need the numpy.array(acc_set).mean() and the code can be simplified.

[daming-lu]

Thanks for the changes 👍