[BUG] Task progress bar format is not uniform

[zhongchun]

Describe the bug The task progress bar numbers are not aligned, and the number of digits of the data is inconsistent. We i run the following codes:

import mars
import mars.tensor as mt
import mars.dataframe as md


mars.new_session()
df = md.DataFrame(
    mt.random.randint(1, 100, size=(1000, 10), chunk_size=(130, 5)),
    columns=list("ABCDEFGHIJ"),
)
df.describe().execute()

it prints:

   1%|▋                                                            | 1.1544011544011543/100 [00:02<01:26,  1.15it/s]
 20%|████████████▏                                                | 20.057720057720058/100 [00:03<00:06, 11.56it/s]
 35%|█████████████████████▍                                        | 34.63203463203463/100 [00:04<00:05, 12.71it/s]
 46%|████████████████████████████▎                                | 46.320346320346324/100 [00:05<00:04, 12.31it/s]
 59%|████████████████████████████████████▌                         | 58.87445887445888/100 [00:06<00:03, 12.39it/s]
 72%|██████████████████████████████████████████████                  | 72.005772005772/100 [00:07<00:02, 12.64it/s]
 85%|████████████████████████████████████████████████████▊         | 85.28138528138528/100 [00:08<00:01, 12.85it/s]
100%|██████████████████████████████████████████████████████████████████████████| 100.0/100 [00:09<00:00, 10.89it/s]

To Reproduce To help us reproducing this bug, please provide information below:

1. Your Python version: 3.7.9
2. The version of Mars you use: master
3. Versions of crucial packages, such as numpy, scipy and pandas
4. Full stack of the error.
5. Minimized code to reproduce the error.

Expected behavior A clear and concise description of what you expected to happen.

Additional context Add any other context about the problem here.