[BUG] Failover remote functions failed

Nobody has claimed this yet.

**Describe the bug**

Failover remote functions failed.

**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 protobuf
4. Full stack of the error.
5. Minimized code to reproduce the error.

```
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/graph.py in _execute_graph()

/home/admin/work/_public-mars-0.4.2.zip/mars/utils.py in _wrapped()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/graph.py in analyze_graph()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/graph.py in _assign_initial_workers()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.result()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult._raise_exception()

/opt/conda/lib/python3.7/site-packages/gevent/_compat.py in reraise()

/opt/conda/lib/python3.7/site-packages/gevent/threadpool.py in __run_task()

/usr/local/lib/python3.7/site-packages/mars/actors/pool/gevent_pool.pyx in mars.actors.pool.gevent_pool.GeventThreadPool._wrap_watch.inner()
126 gevent.spawn(check, event)
--> 127 result = fn(*args, **kwargs)
128 event.set()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/graph.py in _do_assign()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/graph.py in assign_operand_workers()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/analyzer.py in calc_operand_assignments()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/analyzer.py in _iter_assignments_by_transfer_sizes()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/analyzer.py in ()

/home/admin/work/_public-mars-0.4.2.zip/mars/scheduler/analyzer.py in ()

AttributeError: 'NoneType' object has no attribute 'chunk_size'

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

ExecutionFailed Traceback (most recent call last)
/home/admin/work/_public-mars-0.4.2.zip/mars/promise.py in _wrapped()

/home/admin/work/_public-mars-0.4.2.zip/mars/worker/calc.py in ()

/home/admin/work/_public-mars-0.4.2.zip/mars/worker/calc.py in _start_calc()

/home/admin/work/_public-mars-0.4.2.zip/mars/worker/calc.py in _calc_results()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.result()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult.get()

/opt/conda/lib/python3.7/site-packages/gevent/_gevent_cevent.cpython-37m-x86_64-linux-gnu.so in gevent._gevent_cevent.AsyncResult._raise_exception()

/opt/conda/lib/python3.7/site-packages/gevent/_compat.py in reraise()

/opt/conda/lib/python3.7/site-packages/gevent/threadpool.py in __run_task()

/usr/local/lib/python3.7/site-packages/mars/actors/pool/gevent_pool.pyx in mars.actors.pool.gevent_pool.GeventThreadPool._wrap_watch.inner()
126 gevent.spawn(check, event)
--> 127 result = fn(*args, **kwargs)
128 event.set()

/home/admin/work/_public-mars-0.4.2.zip/mars/executor.py in execute_graph()

/home/admin/work/_public-mars-0.4.2.zip/mars/executor.py in execute()

/opt/conda/lib/python3.7/concurrent/futures/_base.py in result()

/opt/conda/lib/python3.7/concurrent/futures/_base.py in __get_result()

/opt/conda/lib/python3.7/concurrent/futures/thread.py in run()

/home/admin/work/_public-mars-0.4.2.zip/mars/executor.py in _execute_operand()

/home/admin/work/_public-mars-0.4.2.zip/mars/executor.py in handle()

/home/admin/work/_public-pyodps-0.9.3.zip/odps/mars_extension/core.py in wrapper()

/home/admin/work/_public-mars-0.4.2.zip/mars/remote/core.py in execute()

in process_shap()
54
---> 55 return mr.ExecutableTuple(results).execute()
56

/home/admin/work/_public-mars-0.4.2.zip/mars/core.py in execute()

/home/admin/work/_public-mars-0.4.2.zip/mars/core.py in execute()

/home/admin/work/_public-mars-0.4.2.zip/mars/session.py in run()

/home/admin/work/_public-mars-0.4.2.zip/mars/session.py in run()

ExecutionFailed: '\'\\\'\\\\\\\'"\\\\\\\\\\\\\\\'Graph execution failed.\\\\\\\\\\\\\\\'"\\\\\\\'\\\'\''

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

ExecutionFailed Traceback (most recent call last)
in
55 return mr.ExecutableTuple(results).execute()
56
---> 57 print(mr.spawn(process_shap, retry_when_fail=False).execute().fetch())

/usr/local/lib/python3.7/site-packages/mars/core.py in execute(self, session, **kw)
368
369 # no more fetch, thus just fire run
--> 370 session.run(self, **kw)
371 # return Tileable or ExecutableTuple itself
372 return self

/usr/local/lib/python3.7/site-packages/mars/session.py in run(self, *tileables, **kw)
426 tileables = tuple(mt.tensor(t) if not isinstance(t, (Entity, Base)) else t
427 for t in tileables)
--> 428 result = self._sess.run(*tileables, **kw)
429
430 for t in tileables:

/usr/local/lib/python3.7/site-packages/mars/web/session.py in run(self, *tileables, **kw)
185 timeout_val = min(check_interval, timeout - time_elapsed) if timeout > 0 else check_interval
186 try:
--> 187 if self._check_response_finished(graph_url, timeout_val):
188 break
189 except KeyboardInterrupt:

/usr/local/lib/python3.7/site-packages/mars/web/session.py in _check_response_finished(self, graph_url, timeout)
144 exc_info = pickle.loads(base64.b64decode(resp_json['exc_info']))
145 exc = exc_info[1].with_traceback(exc_info[2])
--> 146 raise ExecutionFailed('Graph execution failed.') from exc
147 else:
148 raise ExecutionFailed('Graph execution failed with unknown reason.')

ExecutionFailed: 'Graph execution failed.'
```

Start with the traceback locations in mars/scheduler/analyzer.py, especially _iter_assignments_by_transfer_sizes, and mars/scheduler/graph.py, including _assign_initial_workers. Reproduce the reported remote-function failover failure if possible and trace why a None value reaches chunk_size; done means the failure is resolved and the failover path no longer raises this error.

Written by the indexing model from the issue text.