[ray] refine mars on ray usability

[chaokunyang]

What do these changes do?

Refine mars on ray usability and documents.

Example1

import mars
import mars.dataframe as md
import mars.tensor as mt
session = mars.new_ray_session(cluster_name='abc', worker_num=2)
mt.random.RandomState(0).rand(1000_0000, 5, chunk_size=100_0000).sum().execute()
df = md.DataFrame(
    mt.random.rand(1000_0000, 4, chunk_size=500_0000),
    columns=list('abcd'))
print(df.sum().execute())

Example2

Create a mars cluster:

import mars.dataframe as md
import mars.tensor as mt
from mars.deploy.oscar import new_cluster_in_ray, new_ray_session
cluster = new_cluster_in_ray(cluster_name='test', worker_num=2)

then connect it in the client:

session = new_ray_session(address = cluster.address)

Related issue number

#2112

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass, see here for how to run them

[wjsi]

Please remove unused imports according to messages by flake 8 (https://dev.azure.com/mars-project/mars/_build/results?buildId=1934&view=logs&jobId=435ca956-9126-505f-566f-fff31072e2ba&j=435ca956-9126-505f-566f-fff31072e2ba&t=456eced4-5f9f-56bd-1d85-873f530e72f3).

[chaokunyang]

[wjsi]

LGTM

[qinxuye]

LGTM