[BUG] DASK-on-Mars failed when compute multiple objects

[qinxuye]

Describe the bug

DASK-on-Mars failed when compute multiple objects.

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

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

import dask
import mars
from mars.contrib.dask import mars_scheduler

def add(x: int):
    return x + 1


mars.new_session()
xs = [dask.delayed(add)(i) for i in range(10)]
dask.compute(xs, scheduler=mars_scheduler)

Output:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-12-531953a29902> in <module>
      4 
      5 xs = [dask.delayed(add)(i) for i in range(10)]
----> 6 dask.compute(xs, scheduler=mars_scheduler)

~/miniconda3/lib/python3.7/site-packages/dask/base.py in compute(*args, **kwargs)
    567 
    568     results = schedule(dsk, keys, **kwargs)
--> 569     return repack([f(r, *a) for r, (f, a) in zip(results, postcomputes)])
    570 
    571 

~/miniconda3/lib/python3.7/site-packages/dask/base.py in repack(results)
    448         dsk = repack_dsk.copy()
    449         dsk[collections_token] = quote(results)
--> 450         return simple_get(dsk, out)
    451 
    452     return collections, repack

~/miniconda3/lib/python3.7/site-packages/dask/core.py in get(dsk, out, cache)
    149     for key in toposort(dsk):
    150         task = dsk[key]
--> 151         result = _execute_task(task, cache)
    152         cache[key] = result
    153     result = _execute_task(out, cache)

~/miniconda3/lib/python3.7/site-packages/dask/core.py in _execute_task(arg, cache, dsk)
    119         # temporaries by their reference count and can execute certain
    120         # operations in-place.
--> 121         return func(*(_execute_task(a, cache) for a in args))
    122     elif not ishashable(arg):
    123         return arg

IndexError: list index out of range

[qinxuye]

@loopyme Could you please check what went wrong?

[loopyme]

Work in progress