[example] fix: fix math circular dependency

[eric-haibin-lin]

Avoid using file name math since it may lead to circular dependency due to file name conflict

Traceback (most recent call last):
  File "/teamspace/studios/this_studio/verl/examples/data_preprocess/gsm8k.py", line 20, in <module>
    import datasets
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/datasets/__init__.py", line 17, in <module>
    from .arrow_dataset import Dataset
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/datasets/arrow_dataset.py", line 23, in <module>
    import math
  File "/teamspace/studios/this_studio/verl/examples/data_preprocess/math.py", line 21, in <module>
    from verl.utils.hdfs_io import copy, makedirs
  File "/teamspace/studios/this_studio/verl/verl/__init__.py", line 22, in <module>
    from .protocol import DataProto
  File "/teamspace/studios/this_studio/verl/verl/protocol.py", line 19, in <module>
    import numpy as np
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/numpy/__init__.py", line 157, in <module>
    from . import random
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/numpy/random/__init__.py", line 180, in <module>
    from . import _pickle
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/site-packages/numpy/random/_pickle.py", line 1, in <module>
    from .mtrand import RandomState
  File "numpy/random/mtrand.pyx", line 1, in init numpy.random.mtrand
  File "bit_generator.pyx", line 40, in init numpy.random.bit_generator
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/secrets.py", line 19, in <module>
    from random import SystemRandom
  File "/home/zeus/miniconda3/envs/cloudspace/lib/python3.10/random.py", line 49, in <module>
    from math import log as _log, exp as _exp, pi as _pi, e as _e, ceil as _ceil
ImportError: cannot import name 'log' from partially initialized module 'math' (most likely due to a circular import) (/teamspace/studios/this_studio/verl/examples/data_preprocess/math.py)