Add warning for dataloader incompatable upgrade

[heavengate]

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Describe

Add warning for DataLoader output data format incompatable upgrade when dataset only contains single filed.

test code:

import numpy as np
import paddle
from paddle.io import DataLoader, Dataset

class RandomDataset(Dataset):
    def __getitem__(self, idx):
        return np.random.random((2, 3)).astype('float32')

    def __len__(self):
        return 10

dataset = RandomDataset()
loader = DataLoader(dataset, batch_size=1)
for data in loader():
    print(data)

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[jzhang533]

python2 和 python 3.5都已经drop了，这里可以不需要再顾忌collections.abc了吧。

[heavengate]

Done, thanks!

[CLAassistant]

All committers have signed the CLA.

[paddle-bot-old]

Sorry to inform you that 1e89504's CIs have passed for more than 7 days. To prevent PR conflicts, you need to re-run all CIs manually.

[chenwhql]

LGTM