user need to set feed order for Trainer.train and Trainer.test

[jacquesqiao]

Two reasons:

1. I found that the order of
   

   Paddle/python/paddle/fluid/trainer.py

   Line 281 in 6c32052

   var for var in program.global_block().vars.itervalues()

is unstable, it will turn out in different order on different machine.

2. the order may change if user change it's train program, for example
   

   Paddle/python/paddle/fluid/tests/book/high-level-api/fit_a_line/test_fit_a_line.py

   Lines 41 to 48 in 6c32052

   	def linear():
   	y = fluid.layers.data(name='y', shape=[1], dtype='float32')
   	y_predict = inference_program()
   	loss = fluid.layers.square_error_cost(input=y_predict, label=y)
   	avg_loss = fluid.layers.mean(loss)
   	return avg_loss

if user write the code as:

def linear():
    y_predict = inference_program()
    y = fluid.layers.data(name='y', shape=[1], dtype='float32')
   
    loss = fluid.layers.square_error_cost(input=y_predict, label=y)
    avg_loss = fluid.layers.mean(loss)

    return avg_loss

the order of data layer will change.

[daming-lu]

Related issues are:
#10426
#10588

[tonyyang-svail]

LGTM.

[daming-lu]

Note this change could cause failure on certain tests as those tests might use default feed_order. Will fix those tests correspondingly.