[BUG] sort_values failed after using dropna

[hoarjour]

Describe the bug
when I try to use sort_values(ignore_index=True) after dropna, it raises TypeError:

a = md.Series([1,3,2,np.nan,np.nan])
a.dropna().sort_values(ignore_index=True).execute()

but I can do it in pandas:

b = pd.Series([1, 3, 2, np.nan, np.nan])
b.dropna().sort_values(ignore_index=True)

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

1. Your Python version: 3.8.0
2. The version of Mars you use: 0.6.11
3. Versions of crucial packages, such as numpy, scipy and pandas: pandas: 1.1.3
4. Full stack of the error.

ValueError                                Traceback (most recent call last)
c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\dtypes\common.py in ensure_python_int(value)
    170     try:
--> 171         new_value = int(value)
    172         assert new_value == value

ValueError: cannot convert float NaN to integer

The above exception was the direct cause of the following exception:

TypeError                                 Traceback (most recent call last)
<ipython-input-18-f7e878c753c1> in <module>
      1 a = md.Series([1,3,2,np.nan,np.nan])
----> 2 a.dropna().sort_values(ignore_index=True).execute()

c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\dataframe\sort\sort_values.py in series_sort_values(series, axis, ascending, inplace, kind, na_position, ignore_index, parallel_kind, psrs_kinds)
    317                              parallel_kind=parallel_kind, psrs_kinds=psrs_kinds,
    318                              output_types=[OutputType.series], gpu=series.op.is_gpu())
--> 319     sorted_series = op(series)
    320     if inplace:
    321         series.data = sorted_series.data

c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\utils.py in _inner(*args, **kwargs)
    454         def _inner(*args, **kwargs):
    455             with self:
--> 456                 return func(*args, **kwargs)
    457 
    458         return _inner

c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\mars\dataframe\sort\sort_values.py in __call__(self, a)
     97         assert self.axis == 0
     98         if self.ignore_index:
---> 99             index_value = parse_index(pd.RangeIndex(a.shape[0]))
    100         else:
    101             if isinstance(a.index_value.value, IndexValue.RangeIndex):

c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\indexes\range.py in __new__(cls, start, stop, step, dtype, copy, name)
    100             raise TypeError("RangeIndex(...) must be called with integers")
    101 
--> 102         start = ensure_python_int(start) if start is not None else 0
    103 
    104         if stop is None:

c:\users\hoa'r'jou'r\appdata\local\programs\python\python38\lib\site-packages\pandas\core\dtypes\common.py in ensure_python_int(value)
    172         assert new_value == value
    173     except (TypeError, ValueError, AssertionError) as err:
--> 174         raise TypeError(f"Wrong type {type(value)} for value {value}") from err
    175     return new_value
    176 

TypeError: Wrong type <class 'float'> for value nan

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

Additional context
Add any other context about the problem here.

[hekaisheng]

Please copy-paste your code and error message instead of screenshots.

[wjsi]

Can be fixed by parsing pd.RangeIndex(-1) when size of certain dimension is unknown.